I'm a New Yorker, hate the heat, hate the humidity, so I thought I was going to be miserable in Florida in August . . .
Ok it IS HOT, and HUMID. It's hot and humid all day, and all night, it doesn't cool down in FL at night the way it does here in NY, so it's boiling hot, and way too humid first thing in the morning, as soon as you step foot out of the hotel.
The good news is, it didn't bother me anywhere near as much as I expected it to. Everything is flat there, not too much in the way of hills, or stairs to climb, so the walking about is pretty easy. Most everything there is air conditioned, so you get plenty of opportunities to escape the heat as well.
Drink plenty of fluids, and get into the A/C when ever you have a chance. When you start to feel too hot, or worn down, sit down for a bit, have a nice cold drink, and in no time you'll be ready to go again. Make meals a nice long break, and soak up the cool air. We go pretty much from rope drop to park closing every day, and have no problems, even in the August heat.