If I’m doing a competitive 12 hour time trial 132 to 145 bpm is what I target. Based on nothing other than experience. If I try really hard I can get my hr up to 190 bpm.
So I guess 70% to 75% of max HR is something I can hold for a good long while.
If you want some anecdotal data, take a look at the last leg of the Dirty Kanza 200+ race. Lots of racers will just have their power data up on Strava & you can track them down by looking at DK historic results. If you can hold 170+ watts over that last leg you’re pretty good. 180+ you might be elite. 200+ & you’re probably Ted King.
But that’s still a lot fewer miles than teamkennyg’s ~350 endurance odyssey! Sounds like fun. If you want to see where your base is at, why not just go out and do a hundo? That’s the way I do it…