Some years ago, in much better shape I could go out and ride 60+ mile days in 100* high humidity GA summer heat. Generally had few problems almost in spite of nutrition or water intake. I found that once I passed the 70-ish mile mark I would commonly be plagued with leg and particularly glute cramps.
Members of the club/group I rode with commonly discussed that it was a combination of pre-hydration, as well as timing your stops to refuel early.

It was very common for the routes in which I would be riding that mileage that the first rest stop would be swarmed and would start my downhill trek towards not finishing. I have not ever been able to go full century.