Did you ride on Saturday or Sunday? My guess is you rode on Saturday, and if your weather was like mine on Saturday, it doesn't surprise me you were tired at the end. It wasn't just 90 degrees but also extremely high humidity.
Your heart rate shows a pretty linear increase over the duration of the ride, that I'm assuming wasn't caused by increased intensity of riding. If that is the case you could have potentially been getting too hot towards the end of the ride, which caused the increased heart rate and fatigue. It may also have been dehydration, but it sounded like you drank a fair amount of water. With the super high humidity your body probably wasn't cooling off adequately. At least that is my wild a$$ guess. I'm no doctor, but I did study exercise physiology in college 15 years ago
Averaging over 14 mph over 60 miles in 90 degree heat and humidity so thick you could cut it with a knife is no task most out of shapers would accomplish.