Triathlon to Marathon: Your Engine Is Ready, Your Legs Need Six More Weeks

If you just finished a 70.3 or an IRONMAN and there is a fall marathon on your calendar, you are in an unusually good position. The aerobic system you spent all summer building transfers, and the research is reassuring on that point. What doesn't transfer is the thing your legs have to absorb — and the tissue that has to adapt runs on a much slower clock than the engine driving it.

This is one of the best-timed opportunities in age-group endurance sport and one of the least-served. Chicago is October 11. Marine Corps is October 25. New York is November 1. Indianapolis Monumental is November 7. Every one of those sits five to nine weeks from now — and almost every marathon plan you can find is built for eighteen weeks and assumes you are starting from a running base, not from a summer of swim-bike-run.

So here is the version written for where you actually are.

The Engine Really Does Transfer

Start with the encouraging part, because it is better evidenced than most people assume.

A 2026 systematic review and meta-analysis in Frontiers in Sports and Active Living pooled seven randomized trials comparing cycling and running training, with 143 participants ranging from untrained to competitive runners. On running performance, the difference between training modes came out at a Hedges' g of 0.02 — effectively zero, with a p-value of 0.88. On treadmill VO2max the estimate was −0.32 and non-significant. The authors' own conclusion is that current evidence does not show clear improvements or decrements in sport-specific VO2max or running performance from cross-training.

Read that carefully, because it cuts in a useful direction. It does not say cycling makes you a better runner. It says the aerobic work you did on the bike did not cost you, which is exactly the reassurance a triathlete needs when they look at a marathon plan and see mileage they have not been running. With 143 participants and confidence intervals spanning roughly ±0.6, this cannot rule out real effects in either direction — but the panic that your summer was wasted is not supported.

There is a second finding worth knowing, from a 2009 review in Sports Medicine by Millet and colleagues on cycling and running in triathletes: transfer appears to run more strongly from running to cycling than the other way around, and both central fatigue and loss of maximal strength are more pronounced after prolonged running than after prolonged cycling. Running is the more mechanically demanding of the two, and the physiology reflects it.

Athlete Takeaway: Trust the engine. Your aerobic base from a summer of triathlon training is genuinely there, and you do not need to rebuild it from scratch in six weeks.

What Doesn't Transfer Isn't Your Muscles — It's Impact Cycles

Here is where the popular version of this article usually goes wrong, and it is worth correcting because the wrong mechanism leads to the wrong plan.

The story you will read elsewhere is that a cyclist's muscles are naive to eccentric loading, so a triathlete's legs get shredded by running. That specific claim does not hold up. A 2013 study in the European Journal of Applied Physiology put ten elite road cyclists, ten distance runners and ten untrained participants through a hundred submaximal eccentric knee extensions. Both athlete groups were clearly protected relative to the untrained group — and the cyclists were not meaningfully worse off than the runners. Maximal voluntary contraction dropped 25% immediately in the cyclists versus 20% in the runners, and by 48 hours the two groups had essentially converged at 6% and 5%. Trained muscle is trained muscle.

The real gap is not in your quadriceps. It is in how many times your skeleton has been loaded by ground contact. A summer of triathlon accumulates enormous aerobic hours with comparatively few impact cycles, because swimming and cycling deliver almost none. And bone responds to impact cycles, at a pace that has nothing to do with how your engine feels.

The reference work here is a 2021 review in Current Osteoporosis Reports by Warden, Edwards and Willy on preventing bone stress injuries in runners. The timeline it lays out is the whole argument of this article:

• Osteoclast activation and resorption take approximately four weeks.

• Replacement with new bone can take three months, and up to a year for full mineralization.

• Critically, resorption creates porosity, which reduces the tissue's fatigue resistance — so there is a window during remodeling where the bone is temporarily less able to tolerate load, not more.

• Bone stress injuries tend to appear roughly three to four weeks after a significant increase in load, not during it.

Two further numbers from the same review make the sensitivity vivid. A 10% increase in tissue strain roughly halves the number of loading cycles bone can take before failure, and in laboratory work microdamage accumulation scales with strain magnitude by an exponent of about 17. Bone does not respond to load in proportion — it responds steeply.

Now put that beside the adaptations you actually feel. Plasma volume expands within hours to days of training and by a substantial margin — a 1992 review in Sports Medicine puts it at 9–25%, or 300–700 mL. Mitochondrial enzyme activity rises within roughly 7–10 days. Tendon is slower again: a 2015 meta-analysis in Sports Medicine Open covering 27 studies and 264 participants found that 35 of 37 training interventions ran 8–14 weeks, and that the gains were mostly in stiffness and material properties rather than size — cross-sectional area barely moved.

Days for blood volume. A week or two for mitochondria. Eight to fourteen weeks for measurable tendon change. Four weeks to a year for a single bone remodeling cycle. Your engine and your chassis are on completely different clocks, and the engine is the one that talks to you.

A note on how we are reasoning here: each of those time constants is individually well sourced, but no study has measured aerobic and musculoskeletal adaptation head-to-head in the same athletes across a six-to-ten week block. The ordering is solid; treating the comparison as a precise ratio would be overreaching, and we are not going to do that.

Athlete Takeaway: The limiter is impact exposure, not muscle. Plan the next six weeks around how many times your legs hit the ground, not around how your breathing feels.

What This Looks Like in Triathletes Specifically

Two findings move this from mechanism to something closer to evidence.

The first is a 2019 study in Sports examining stress fractures in elite German triathletes. Among 86 athletes, 23% reported having had a stress fracture, with an annual incidence of 4.8%. Every site was in the lower extremity — metatarsal 52%, femur or femoral neck 19%, tibia 10%. The pattern in how they happened is the part worth sitting with: injured athletes were running more than uninjured ones (4.6 versus 3.7 hours per week), 57% had increased their training before onset, primarily by increasing distance, and 90% reported the pain starting during or after running. In triathletes, this is a running-limb, running-onset, distance-increase phenomenon.

The honest caveat: that cohort had a mean age of 17.5 and the design was retrospective and cross-sectional, so it transfers to a 45-year-old age-grouper directionally rather than precisely.

The second is a 2012 systematic review in BMC Medicine covering 31 studies on cycling and bone health. It found that roughly two-thirds of professional and master road cyclists could be classified as osteopenic, with lower lumbar spine and femoral neck density than runners, and concluded that road cycling does not appear to confer meaningful bone-building benefit. The finding worth carrying forward is the constructive one: cycling combined with running was protective relative to cycling alone. The running is not the problem. The rate of arriving at it is.

Athlete Takeaway: If you have been bike-dominant this year, your ramp matters more than your weekly total. Add running consistently rather than suddenly, and it works in your favor.

The 10% Rule Is Not Actually a Rule

Almost everyone reaching for a marathon off a triathlon build uses the ten-percent-per-week guideline. It deserves a closer look than it usually gets.

It was tested properly. A 2008 randomized controlled trial in the American Journal of Sports Medicine put 532 novice runners into either a graded 13-week program built on the principle or a standard 8-week program. Injury rates came out at 20.8% and 20.3% — statistically indistinguishable (p = 0.90). The authors concluded the graded program was not preventive. Warden's review reaches the same place from the mechanistic side, noting that individual tolerance varies far too much for one rule to apply uniformly.

The successor concept, acute-to-chronic workload ratio, has not fared much better. Two 2020 methodological critiques — Impellizzeri and colleagues in IJSPP, and Wang and colleagues in Sports Medicine — identified mathematical coupling, spurious correlation and a set of statistical problems severe enough that both papers explicitly discourage its use for training recommendations. If a coach or an app is telling you your ACWR is in a safe zone, that number is doing less than it appears to.

So what should you use instead? The most useful recent finding reframes the question from weekly volume to single sessions, which happens to be exactly right for a triathlete.

A 2025 study in the British Journal of Sports Medicine, drawn from Garmin data on 5,205 runners across 588,071 sessions over eighteen months, compared each run against that athlete's longest run in the previous 30 days. Injury rates rose once a single session exceeded that longest run by more than 10%: a hazard rate ratio of 1.64 for 10–30% over, 1.52 for 30–100% over, and 2.28 for more than double. The outcome measured was overuse running injury generally rather than bone stress injury specifically, but the signal is clear.

Why this matters so much for you: a triathlete coming off a 70.3 build has large 30-day aerobic hours and a small 30-day longest run. Those two numbers point in opposite directions, and it is the second one that governs what your next long run should be. Your fitness says you can run 16 miles. Your last month says your longest run was 8. The 30-day longest run is the number to build from.

Athlete Takeaway: Find your longest single run in the last 30 days. That is your reference point — grow the long run from there in modest steps, regardless of what your aerobic fitness says you could survive.

How to Build the Bridge

With that framing, the plan mostly writes itself. A few principles do the heavy lifting.

Run frequency before run distance. More frequent, shorter runs deliver impact exposure at lower per-session strain than the same volume packed into two long efforts. Warden's review notes that more than 90% of bone's mechanosensitivity is restored with four to eight hours of rest between bouts, which is a real argument for spreading load across days rather than concentrating it.

Grow the long run from the 30-day reference, not from the plan. If a downloaded eighteen-week plan says your next long run is 16 miles and your longest in the past month is 8, the plan is describing an athlete you are not yet. Step up from where you are.

Keep some bike. This is the underrated move. Your aerobic system responds to load regardless of modality, and the bike lets you hold aerobic volume while the run frequency climbs — which is precisely the combination the cycling-and-bone literature found protective. Dropping cycling to zero the moment you switch focus adds run load and removes aerobic support at the same time.

Keep lifting. Whatever strength work you were doing through the season should continue through this block, not stop for it. Our guide to strength training during race season covers how little it takes to hold what you have.

Respect the three-to-four week delay. Because bone stress injuries tend to surface weeks after the load change that produced them, the run week that eventually causes trouble usually feels fine at the time. This is the single strongest argument for building conservatively in weeks one and two of your bridge, when your engine is loudest and your legs have the least accumulated running behind them.

Treat new bony pain as information. Localized, point-specific pain in the shin, foot or hip that gets worse as a run continues is a different signal from general muscular soreness, and it is worth getting looked at rather than trained through. Our article on returning to running after time off covers this ground in more depth.

One thing we are not going to give you is a minimum number of runs per week required to maintain running durability. We looked for it. The evidence does not exist — there is good work on maintaining strength and aerobic capacity on reduced frequency, and nothing usable on bone and tendon specifically. Anyone giving you a precise number for that is giving you coaching convention, and it should be labeled as such.

Athlete Takeaway: Build frequency first, grow the long run from your real 30-day maximum, keep some cycling and keep lifting. Be most conservative in the first two weeks, when your engine is loudest.

When the Honest Answer Is "Run It Next Spring"

Sometimes the right call is to move the goal, and that is worth saying plainly rather than burying.

If your longest run in the last month is under an hour, if you have been managing a niggle in a foot, shin or hip through the back half of your triathlon season, or if the marathon is under six weeks away and you would need to double your long run to get there — the bridge is asking for more than the calendar can safely give. That is not a fitness verdict. It is arithmetic about tissue.

The good version of that decision: hold the aerobic fitness you have, put a half marathon or a shorter race on the fall calendar instead, and point the whole thing at a spring marathon with a proper runway. Houston in mid-January and the Texas spring races give you a genuinely better platform than a compressed six weeks — and we have already laid out how that build works in your 2027 Houston Marathon training plan.

An athlete who arrives at a spring marathon healthy, off a winter of consistent running, will run faster than the same athlete who forced a fall one. That is not a consolation prize. It is the better race.

Athlete Takeaway: If closing the gap would require doubling your long run inside six weeks, point at a spring marathon instead. You will run faster off a real runway than a rushed one.

The Bottom Line

Coming off a triathlon season is one of the best possible starting points for a marathon. The engine is built, the discipline is there, and the aerobic work you did on the bike did not cost you anything on the run — the meta-analysis is reassuring on exactly that point.

The whole task of the next six to ten weeks is letting the rest of you catch up to it. Bone runs on a four-week-to-a-year clock and tendon on an eight-to-fourteen-week one, while the fitness that makes you feel ready responded in days. Add frequency before distance, grow the long run from what you have actually been running rather than from what a plan says, and keep enough bike and enough lifting to support the whole thing.

Do that, and the fall marathon is a genuinely good idea. The transition from triathlon to marathon is one of the most enjoyable pivots in endurance sport, and it rewards patience in a way very few six-week blocks do.

If you would like a second set of eyes on whether your bridge is realistic, we would love to help. Getting the ramp right is exactly the kind of question that benefits from someone who has seen a few hundred of them. Set up a call with us now, and we will help you be ready for your best race day.