TrainerRoad has an enormous set of rider data that could be used to provide insight in what intensity is sustainable over various durations, whether it be training, racing or long endurance challenges.
When planning for a 3-5 hour race, a long gran fondo (La Marmotte, Maratona Dles Dolomites), a seriously long race (Tour du Mont Blanc, Dirty Kansa) or an everesting attempt (>8,848 vertical metres), I have noticed that several members of this forum have asked for input, sharing of experiences and actual numbers (TSS and IF specifically). Regardless of your FTP, I would assume that the maximum you could achieve in such attempts can be measured in TSS or IF.
E.g., generally speaking: 100 TSS/1.00 IF is close to the maximum a rider could hold for 60 minutes, whereas a 5 minute VO2max interval may be 10 TSS/1.10 IF (per repeat), and an everesting attempt may last for 14 hours and consist of 500 TSS/0.70 IF.
I have my numbers and experiences that I base my planning on, but it would be useful if Trainerroad could provide some data based insights and recommendations here, perhaps graphically (x-axis being IF and Y-axis being duration) with the option to filter on gender, age or other dimensions (if there are significant differences). This would be very useful in two aspects:
Firstly, for planning purposes, if you know that TR data suggests than an IF of 0.85 is what is generally possible to hold for 3 hours, then you know how hard you should step on the pedal for the upcoming race without blowing up (“I see that last year I rode race X for 2:55 with an IF of 0.75, perhaps I should challenge myself to go harder this year”, or “last time I rode at an IF of 0.9 for the first 2 hours, no wonder I blew up”).
Secondly, it might be useful to indicate wether your FTP estimation is correct or should be adjusted. E.g. if you’ve ridden a 5 hour ride and managed to log an IF of 0.95, then perhaps your FTP is set too low and you are ripe for a ramp test?