The short answer
You probably overbiked your IRONMAN if two or more of these are true: your normalised power sat above a level you have genuinely held for four hours or more, your power faded more than about 10% late in the ride, your heart rate drifted more than 16% away from power across the day, and your marathon started near target then decayed progressively rather than being slow from the first kilometre.
On the full distance especially, an overbike and a fuelling shortfall look almost identical by the time you are walking. The pattern that separates them is when the decay starts — a bike that was too hard shows up as a marathon that holds for 10 to 15 km and then gives way, not one that never had legs at all.
The full-distance numbers
These differ from the 70.3 set, and using half-distance thresholds on a full is one of the commonest self-diagnosis errors. A five-to-six hour ride simply behaves differently:
- Intensity factor 0.68-0.73 is the target band. At 0.75 and above we treat it as an overbike - notice that is below the 70.3 target band entirely.
- Power-to-heart-rate drift under 10% is normal over this long, against 5% for a half. Over 16% is the real signal.
- Fade under 5% is tolerable across five hours. Over 10% is significant.
- On the marathon, a split within 3% is excellent and within 5% is acceptable - a touch more forgiving than the half, because an even full-distance marathon is genuinely hard to achieve.
The 70.3 version of this page carries the half-distance set if you are checking a different race.
Why the full distance is the hardest to judge
Over 112 miles a small overbike does not announce itself. Ten watts too many feels fine for four hours and then costs you the back half of the marathon, long after the bike is over. By the time the run comes apart the cause is hours behind you, which is exactly why memory is such an unreliable witness here.
The advice is the usual: what percentage of FTP, did power fade, was it smooth. All correct, all leaving you to interpret it tired and disappointed. And a high intensity factor on its own does not prove you overcooked it — over a full distance the honest test is whether you rode above a level you have genuinely held for that long before.
What The Debrief actually checks
- Whether your bike power sat above a level you have demonstrably sustained, judged against full-distance targets, not a 70.3 ceiling or an estimated FTP you never touched.
- How your power held across the four to six hours, since on the full distance the fade often creeps in late rather than in a clean second-half split.
- Whether your marathon decayed in the pattern that follows a bike that was too hard, versus fuelling, heat, or a run that went out too quick.
- How your heart rate tracked your power across the day, since decoupling over a long ride is one of the clearer tells that you were spending more than you had.
If the file cannot support the verdict, it will say so. On the full distance especially, an overbike and a fuelling shortfall look almost identical by the time you are walking — the read stays inside what your data can actually prove rather than picking one to sound confident.
No power meter? This still works
Most of the signals above do not need watts. Heart-rate drift needs only a strap, the marathon decay pattern is pure pace, and late fade shows in speed on a comparable stretch of road. A file with heart rate and GPS and no power still gets a read - stated with less certainty on the bike leg, and the report will say so rather than quietly pretending otherwise.
What you get back
A debrief written the way a coach would talk you through it. Not a wall of charts to interpret yourself after the longest day of your season, and not a confident verdict pulled out of thin air. Just what the file shows about how you executed, in plain language, on screen, right after you upload. Here is a full sample on a real 9:12:22 full-distance file.