You are already fitter than you were on the start line at Valencia — and you got there six and a half weeks early. The aerobic job is done. What decides this race now is a single question: can you run 21 km off the bike without your heart rate running away from you? On the evidence of 25 July, not yet.
Three new sessions since the last dossier, then two clear days off. The absorption you were told to take arrived late — but it arrived, and the recovery markers have rebounded hard.
The rebuild, 22 June → 28 July
Chronic training load (CTL, 42-day EWMA of Banister TRIMP) · hover for daily values
The textbook says a 70.3 build wants 8–12 hours a week. You have never done that and you have finished two of these. What matters is your own dose–response — and against that baseline, this is the best-prepared you have ever been at this point.
| Build | Mean h/wk | Biggest week | Bike share | CTL at −6 wk | CTL race day | Result |
|---|---|---|---|---|---|---|
| Valencia 70.3 · Apr 2024 | 4.3 | 6.4 h | 33% | 37.8 | 49.7 | 5:21:08 |
| Weymouth 70.3 · Sep 2025 | 3.4 | 6.4 h | 36% | 24.5 | 61.5 | 4:57 matched legs |
| Vitruvian · now (−6.7 wk) | 5.0 | 7.3 h | 67% | — | 49.6 | ▲ ahead of both |
The sport split is the quiet win. Bike volume has gone from a third of your training to two thirds — exactly what the plan asked for, and the reason the bike has come back so fast.
In June the diagnosis was: swim most detrained, bike the biggest hole, run the retained asset. Five weeks on, that ranking has completely inverted.
Buried in the data is a near-perfect matched pair: the same distance, the same pace, the same flat ground, the same cadence. Only the heart rate differs.
This is not heat and it is not dehydration. Conditions on 25 July were 16 °C; a 1% loss of body mass raises heart rate by only ~3.3 bpm, so fluid loss can account for a handful of beats at most. It is not terrain — 42 m of ascent against zero. It is not form — cadence, ground contact and vertical oscillation are all essentially unchanged. It is aerobic fitness, and the deficit is roughly 30 seconds per kilometre at any given heart rate.
Inside the 25 July run
Two views of the same 11.25 km · pace held flat while heart rate climbed 48 bpm
Aerobic decoupling, every long run since January 2025
Efficiency, second half vs first half · runs ≥ 9 km, continuous, n = 16
The decoupling is a symptom. These are the causes — and every one of them is a scheduling problem rather than a fitness problem, which is why six weeks is enough time.
Your longest run of the block is 61 min. You have not run beyond 61 minutes since 6 September 2025 — 325 days ago. Monthly longest runs in 2026 read: 31, 33, 43, 59, 34, 38, 61 minutes. The research below puts this exactly at the boundary that separates a 3% economy decay from a 6% one.
In four weeks of training there is not one day with a bike and a run on it. The race asks you to run a half marathon off 85 km of cycling, and you have not once rehearsed the first ten minutes of it — the exact window where the neuromuscular pattern is most disrupted.
Block intensity distribution is 60 / 33 / 7 against a planned 78/15/7. Running alone is 43 / 43 / 13 — nearly half of your run time sits in the moderate band. On 5 hours a week that leaves barely 3 hours of genuinely easy aerobic work: not enough base to support the intensity sitting on top of it.
Everything prescribed in the next section traces to one of these. Where a finding is weak or small-sample, it says so.
26 well-trained runners monitored for 6 months, split by whether they did ≥1 session/week over 90 min or capped everything at 70 min. Tested over a 90-minute heavy-domain run. Long-run group: ~3% economy deterioration. Short-session group: ~6%.
5,205 runners, 500,000+ runs, 18 months of Garmin data with weekly injury surveys. Increasing one run beyond your longest of the previous 30 days: +64% overuse injury risk for a 10–30% jump. Weekly mileage change and ACWR showed little to no predictive value.
82,303 marathon runners. Decoupling first appears at 25.2 ± 9.9 km on average; at 33.4 km in the most durable third and 19.1 km in the least — and that group ran 21 minutes slower.
Recreational runners adding plyometrics 3×/week for six weeks improved running economy by 2.3%. A separate trial found 5 minutes of daily double-leg hopping for six weeks significantly improved economy at 12 and 14 km/h.
10 weeks of heavy strength plus plyometrics added to normal running improved running-economy durability across a 90-minute run and increased time-to-exhaustion afterwards. Meta-analyses put strength-driven economy gains at 2–8%.
A 12-minute maximal run off the bike falls ~195 m short of the same run fresh, driven by neuromuscular rather than metabolic fatigue. Athletes who trained more bike-to-run transitions showed less impairment of the stretch-shortening cycle.
Physiological resilience review: prolonged sessions that include bouts at or above race pace, or a progressive rise in intensity as fatigue develops, plus heavy strength and plyometric work.
14 studies. Reduce volume 41–60% (largest effect, SMD −0.77) over 8–14 days (optimal, SMD −1.47), while holding intensity and frequency. Reducing intensity gained nothing.
The strategic instruction is unusual and worth stating plainly: stop building aerobic fitness. You have enough — more than you had racing Valencia. Every remaining week should be spent converting it into run-specific durability.
The long-run ladder
Each step ≤ 10% above your longest run of the previous 30 days · hover for detail
Swim is two laps of the sheltered Whitwell creek; the bike is three tarmac laps of Rutland Water, reported at ~800–850 m of climbing; the run is two flat out-and-backs to Normanton Church on made paths. A flat run course means no terrain to hide a pacing error behind.
| Leg | Conservative | Realistic | Good day | Basis |
|---|---|---|---|---|
| Swim 1.9 km | 37:00 | 35:00 | 33:00 | 1:47/100 m open water, current |
| T1 | 5:00 | 4:00 | 3:30 | — |
| Bike ~85 km | 3:22 | 3:12 | 3:03 | 25.2 / 26.6 / 27.9 km/h · 19 Jul ride matched the profile |
| T2 | 4:00 | 3:00 | 2:30 | — |
| Run 21.1 km | 2:15 | 2:08 | 2:00 | 6:24 / 6:04 / 5:41 per km off the bike |
| Total | 6:23 | 6:02 | 5:42 | Every minute of upside is in the run |
Your standing 5:45 target is a stretch-but-real goal, not a soft one. It needs roughly 5:43/km off the bike — which is your 2024 level (5:32/km) and within reach of six weeks of proper long runs. It does not need any more bike fitness.
Settle immediately. 1:47/100 m — do not chase feet in the first 200 m. Two laps, sheltered water, wetsuit.
HR 144–146 for the first 35 min → 148–150 through the middle → ≤ 152 in the final third. The ceiling is reached late. Over-biking the first 30 minutes costs 3–5 minutes off the run.
80–90 g carb/hr on the bike (2:1 glucose:fructose), 60 g/hr on the run. Rehearse it on every long ride from W7 — gut training cuts GI symptoms 26–47%.
Run 6:15/km for three kilometres regardless of how good you feel, then settle to 5:55–6:05. On this evidence, going out at 5:30 will cost you ten minutes in the back half.