If you really are not often exposed to truly hot weather, you can not only get uncomfortable; you might actually get ill. It is a good idea to take measures to account for that possibility in case it happens, because you don't want to be having to try to hunt down items to help with it when you are in the middle of your trip.
First off, LOTS of sunscreen. You are not used to the angle of the sun that far south in summer, and your skin can absolutely fry in under 2 hours of full summer sun if you don't consistently and properly re-apply. If you are fair-complected and do not normally wear shorts, make it a point to wear them outside in the sunshine at home and "harden" your legs to UV radiation. If you do not, you will be at high risk for getting what is often called "the Disney Rash" on your legs -- polymorphic light eruption. PMLE is an allergic-type reaction, and cannot be prevented with sunscreen only; you need to expose the skin to light over time and take antihistamines at times of unusually high exposure in order to ward it off. It is horrendously itchy and absolutely miserable if you get it, and it does not respond well at all to topical antihistamines. You'll want to prevent it, because you can't cure it in a hurry.Chafing can be a really serious problem in humid weather, especially if you are somewhat overweight. I agree with the PP, bring something to ward that off as well. Monistat anti-chafing cream is popular, but for Orlando in summer I recommend Body Glide; as it is waterproof, which Monistat cream is not. (Note that heat rash just burns, and is different from PMLE, which itches. It's possible to get them both simultaneously, which has happened to me, and which is just about anyone's definition of hell on earth, let me tell you.)