Start with the number that surprises people
Target intensity factor for a full-distance bike leg is 0.68 to 0.73. An overbike starts at 0.75.
Read those against the half-distance figures — target 0.75 to 0.80, overbike at 0.82 — and the shape of the problem appears immediately. The full-distance overbike threshold is at the bottom of the 70.3 target band. A ride that would be a conservative, well-judged half is already too hard for a full.
This is why athletes who step up from 70.3 so often have a worse day on paper despite being fitter. They bring a pacing instinct calibrated on a race that rewards it, and the instinct is not slightly off. It is aimed at a different sport.
Ten watts, four hours, one marathon
The cruelty of the full distance is the delay between the decision and the consequence. Ten watts too many does not feel like anything at ninety minutes. It does not feel like much at three hours. It arrives somewhere around kilometre fifteen of the marathon, by which point the cause is four hours behind you and entirely invisible from where you are standing.
Nothing in the moment corrects for this. The ride feels sustainable because, considered purely as a ride, it is sustainable — you would finish 180 km at that power perfectly well if you could then go home. The bike leg is not an event. It is the opening argument for a marathon, and it has to be paced as one.
A full-distance bike leg ridden correctly should feel conspicuously easy for the first two hours. If it feels appropriately hard early, it is too hard.
The tolerances widen, and that is not permission
Because a full-distance ride runs four to six hours instead of two and a half, the engine judges drift and fade against looser numbers than it uses for a half:
- Power-to-heart-rate drift under 10% is normal, against 5% for a 70.3. Past 16% it is a real signal rather than the day being long.
- Fade under 5% is tolerable across that many hours; over 10% is significant.
- The marathon split is scored within 3% for excellent and 5% for acceptable — a touch more forgiving than the half, because an even full-distance marathon is genuinely hard to produce.
It is worth being clear about why those thresholds are wider. They are wider because some drift over six hours is physiologically normal, not because a full distance is more forgiving of a pacing error. The intensity band moved down while the tolerances moved out. Those two facts point the same way.
Where the fade shows, and why a half-and-half split hides it
On a half you can usually split the bike in two and read the story off the comparison. Over 180 km that often misses it. Full-distance fade tends to creep rather than step: the third hour is a shade under the second, the fourth a shade under the third, and no single split looks alarming while the trend is unmistakable.
Which is why the final stretch is worth isolating on its own. A rider who holds 250 W normalised for 140 km and then produces 200 W for the last 40 has not had a bad patch. They have run out of what they were spending, and the marathon will say so.
Fuelling is tangled into this and cannot be separated afterwards
Every honest full-distance pacing guide has to concede this: ride harder and you oxidise carbohydrate faster, absorb less of what you take in, and arrive at the run both overspent and underfuelled. The two causes are not independent, and by the time you are walking they are indistinguishable from the inside.
They can be told apart in the data, but only by shape. An overbike produces a marathon that holds near target for ten to fifteen kilometres and then grinds down continuously. A fuelling shortfall produces one that is fine and then, within a kilometre or two, is not — often at a repeatable point between 25 and 32 km, frequently behind a bike file that looks perfectly clean. That separation has its own page.
The practical consequence for pacing is simple: riding at the top of the band does not only spend your legs. It also narrows how much fuel you can take on while spending them.
The course decides how much of this you control
The Debrief classifies 31 full-distance courses by elevation gain per kilometre — flat to 8 m/km, rolling to 15 m/km, hilly above — and they split 8 flat, 11 rolling, 12 hilly. The full-distance calendar is noticeably hillier than the 70.3 one, where flat courses are the largest group.
On a flat full distance the discipline is entirely yours: nothing on the road forces a surge, so a high variability index is a decision. On a hilly one the risk inverts. Every climb is an invitation to spend above the band, and over 180 km those invitations arrive dozens of times. Cap the climbs at a ceiling you set before the start, and accept that the average will look modest. The average is not the thing being scored; the marathon is.
A plan that survives contact with the day
- Set the band from power you have genuinely held for four hours or more, not from a stored FTP.
- Ride the first two hours at the bottom of the band, or below it.
- Set a climbing ceiling in advance and do not negotiate with it at kilometre 120.
- Treat drift past 10% as the day telling you the band was wrong for today.
- Judge the last 40 km on their own, not as half of a half.
Afterwards, find out what you actually rode
Upload the file and The Debrief reads the bike and the marathon together: where the power sat against what you have demonstrably sustained, how the fade developed across the day, what the heart rate was doing while it happened, and which of overbiking or fuelling the run pattern actually supports. If the file cannot separate them, it says so rather than picking one. A full sample runs through a real 9:12:22 race.