I try to ride over 70 miles at least once a week. Feel pretty good, until about the 80 mile mark. Have and could do centuries. No problem. Seems at the end of 80 miles the feet and legs feel pretty fatigued. Maybe another break would get me psyched up for another 20. Maybe a power bar or something.
Just wonder, what is your threshold, where you think you have had enough; for one ride at least.