we had our first taste of Olando, Disney, Universal and I-Drive, from 20th August to 2nd September this year. Yes it was hot, even when it was raining, and it was humid all the time. We did mornings in the parks, afternoons back at base for a rest/dip in the pool, then out again in the evening to the parks and dinner. I'm not going to lie, some parks we were fine in, others we struggled (Epcot world showcase has very little shade outside of the buildings and it felt very very hot walking around there in the daytime).
You can't rush, you have to take your time, find air-con shops and shows for a breather, drink lots of water and take breaks if your body is telling you to. We had a couple of half days/days where we just blew off our plans altogether because we needed a complete break.
Just be aware that you'll feel like a sweaty drippy mess, and you'll be fine!