If you've done 80 you should have no problem doing 100.

Drink at least one water bottle/hour. Fill bottles with gatorade or sports drink. Stop every ~90 minutes and eat something with approx 200-300 calories, Cliff Bar, PBJ, Fig Newtons, Pop Tarts, or whatever floats your boat.

Let us know how it goes.

ps Oh yeah, with the heat...START EARLY!