FWIW, I think experience (note this is something different than "training") and mental attitude account for a lot of success at super-long distances. I'm sure there are physiological differences that would explain things at the elite level, but for the rest of us, some combination of mental attitude and experience (been there, done that, and know I can do it again) probably explain who makes for the "faster" rider over longer distance, assuming that the two riders have the same physical conditioning. YMMV.