I grew up not drinking any water all day, and working in the sun. So I was definitely hardened up against the heat. Unfortunately, I seem to have outgrown that strength.
I had a real nice spell of heat exhaustion about 10 years ago. I was climbing a tree, and began to feel weak, so I was hurrying on the removal. On the last cut, I made a bad cut, and ripped the power off the house. Then as I got to the ground, I found that I could no longer stand up without being extremely dizzy and weak. So I laid on my back, drank large quantities of water from the nearby garden hose, but to no avail. Every time I stood up, I was quickly settling back down on the ground.
No amount of water or rest would help. Not until I accepted a Coke from the homeowner did I feel any better. Since then, I have learned that all that nonsense about salt balance is mostly myth: during heat exhaustion you are suffering as much as anything from low blood sugar. ALL the remedies for heat management include sugar.
Since then, I've had quite a few occasions when I felt the heat coming on, I broke immediately for some form of sugar water, and my heat exhaustion symptoms began declining.
I have also found that drinking 8oz of water not less than every 15 minutes lets me blast right through the heat and keep working. I had a friend on manuevers in the desert, the Army made everyone drink not less than a quart per hour. Having tried it out, I agree that it works pretty well. My guys get tired of me pushing the water, but they don't wilt in the heat as badly, either.