Start with what you actually held, not your FTP
Most guides jump straight to intensity factor and a target band — often something like 0.68 to 0.72 for a full distance. The problem is that IF is only as good as the FTP underneath it, and over 112 miles the number that matters is not a lab ceiling but a power you have genuinely held for hours before. A tidy-looking IF against a stale FTP tells you nothing.
The comparison that holds up is against power you have actually sustained for that kind of duration, in training or a previous race. If your ride sat above it, that is the honest signal you went too deep. If it sat at or below it, the marathon problem probably lives somewhere else — most likely fuelling.
Look for the late fade, not just a second-half split
On a 70.3 you split the bike in two. Over a full distance the fade is usually slower and later: the last quarter drifts down while your effort climbs. Watch the final hour especially. A ride that held its power to the end, or eased only slightly, is what good pacing looks like over 180km.
Check how your heart rate tracked your power across the day
Early on, heart rate and power move together. When heart rate keeps climbing through the third and fourth hours while power stays flat or fades, that decoupling is one of the clearer marks of an effort you could not sustain. Over a long day heat and dehydration push the same way, so treat it as a strong clue, not a verdict on its own.
Weigh variability against the day you had
A choppy file with surges out of every corner and coasting on every descent costs you more than the average suggests, and over five hours that tax compounds into the run. A smoother ride at the same average is a better ride. On a hilly or windy course some variability is unavoidable, so judge it against the terrain and conditions, not a flat-course ideal.
Read it against the marathon
The tell that ties it together is the run pattern. An overbike usually shows as a marathon that starts near target and grinds down through the middle miles as the legs empty out, often with a walk creeping in after halfway. A run that was slow from the first mile points more at fuelling, run pacing or fitness than at the bike.
None of these is proof on its own, and on the full distance the margins are smaller — the difference between a well-paced bike and an overbike can be a handful of watts held for five hours. It is a case you build from several signals pointing the same way, which is exactly why it is easy to call wrong and worth being careful about.
Or skip the manual read
All of the above is what The Debrief does when you upload your full-distance race file. It runs the comparison against what you have actually sustained, checks the late fade, the decoupling and the marathon pattern, and gives you the read in plain language. It will also tell you when the file does not support a firm answer, instead of inventing one.