I did a 72 miler on a tandem two weeks ago and felt great at the end. We started
at 6am and finished at about 85F. Last weekend we started a century at 7a and by 72mi it was 94F and I was feeling terrible. Had to stop every ten miles for a 20min cooler at a quickstop after a 45min rest stop at 75mi. Hydration and fueling were adequate as I only lost 2# on the ride. I did finally recover and the last 15mi were not too bad, though the temp topped out at 97F with typical humidities in the 90 range.
Heat does a number on me, has for 30yrs.