Are you a camel? I like to pretend I am. (When I'm not busy pretending I'm a superhero or something.) That means going for long distances in the heat with no extra water. Usually it works for me, but I don't have too many problems with hydration, stomach, etc. In spite of all that, I pay real close attention to what my body is telling me, especially in hot weather.
For most people, the heat is something to deal with. Plan your routes around the locations of water fountains. Carry a bottle or two with you. Go out in the early morning before it gets too hot. BE CAREFUL! The hot weather can cause some problems and you need to be cautious. You've only got about three months of this to deal with -- in Michigan it's usually less than that. So be careful - -we want to see you in October at the starting line!