I’ve found posts here that already kinda explain things, but I’m not sure they are specific to my question.
Rode tonight outside 50 miles in 2:45. Pushed pretty hard a good bit of the ride because the weather’s awesome and I was feeling great. But I don’t know that I was pushing 0.95 IF for 3 hours.
From what I’ve found, IF = NP / FTP
But how does duration play into that? I guess I’m confused on the use of FTP directly on a calculation for a 3 hour ride.
https://www.trainerroad.com/app/career/brad704/rides/81327796
The calculation of IF is independent of duration. It’s simply NP/FTP like you said. Combining IF and duration gives you TSS.
Though if your IF for the ride came out to 0.93 for a 3 hour ride and didn’t feel pretty empty then your FTP is likely higher than you have it set at now.
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In my experience I’ve found that IF and NP can be misleading for rides with a large power variations.
I was having some confusion about my easyish long rides being 0.8-0.85 IF for up to 5 hours, but if I looked at my NP it would be a number that there’s no way I would hold for that time.
I’ve also done a similar ride to you where my IF was 0.95 for 3 hours, it was a hard ride, but it was done a day after an FTP test so it’s unlikely my FTP was that wrong.
I think IF/NP is more about equivalent training stress than equivalent performance, but even then, it’s just an arbitrary calculation and would apply differently to different athletes. e.g. you might expect a crit racer to handle a higher NP that a time trialist relative to their FTP.
This seems like my experience yesterday as well. I also did an ftp test of sorts the day before too.
By Ramp test, my ftp is 249
In my best hour doing the Alp Du Zwift “climb” I avg 239 with NP 241, that was a totally solid effort with zero breaks. 68 minutes total up AlpdZwift.
My long ride yesterday was avg power 170 and NP 232.
Thanks folks who responded, I think I’ve had the epiphany I was looking for. 
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