I second the suggestion of dousing water over your head when you start to overheat. You did the right thing by stopping. DH was suffering from heat exhaustion last July when we did a double century in close to 100 degree heat. The last 30 of the 206 miles was a torture chamber with very little shade. I finally made him pull over and take a good long break. With some added electrolytes pills and plenty of fluid he made it to the finish, but it was scary and unfortunately we didn't have anyone who could come pick him up. The electrolyte pills really saved us when it started to heat up, unfortunately he had stopped taking them and I think that is why the heat got to him.