I find that in the hot weather that I need to try to take a drink about every mile (my Garmin watch buzzes every mile so that is helpful). When I used to run, I couldn't figure out why I would feel so crappy after about 3 hours. Then one day I dragged my bathroom scale with me to the forest preserve where I ran. I weighed myself and then kept track of how many water bottles I went through (a pint is approximately a pound). I weighed 4 pounds less than when I started even though I drank 9 pounds (pints) of fluid. No wonder I felt so miserable.
I hate exercising in the heat. I'm a snow bunny.