re: Heat exhuastion?

You have to hydrate the day/night before you expect to have a day like you describe. Once you reach the point where you even start to feel bad, you won't catch up for a while and that while means in cool climate and drinking lots of fluids with electrolytes. 20 minutes isn't doing crap.

The description you gave about elevated heart rate and dizziness is the precursor to a heat stroke. That's not something to screw around with.

Consider it a lesson learned.