I'm going to chime in here about the electrolytes. You aren't excreting
pure water, are you?
If you lost 12 pounds, you lost a boatload of electrolytes at the same time! (minerals in your bloodstream)

Remember, women die from OVERHYDRATING. happens every year on some marathon, a woman drinks too much liquid and it kills her.

so don't just drink water if you have to ride in that kind of heat.