We don't think FL is all that bad in summer. I think an important aspect is how well acclimated you are to be being outdoors. People who spend all day indoors in AC, which dries the air, and set their thermostat at 70 are going to feel the heat+ humidity much more than people who are frequently outdoors, and/or set their home thermostat at 80 in the summer. If you are often outdoors in the NC afternoon, I don't think FL is all THAT different.
An important aspect is that FL is a peninsula. Mountains run along much of the Eastern USA. Whereas FL is flat and surrounded by water. So mid-day, FL has a period of intense humidity most days. But then often late afternoon there's a downpour. Also, because there are no mountains, after dark, the humidity doesn't get trapped quite the same way it does to the north of FL.
In short, I enjoy FL in summer, but touring enjoyably requires a little strategy. If you try to tough out the afternoon, I find it makes me very sleepy. Best to plan an afternoon break. And just about the time you are thinking of swimming or returning- that's often when a thunderstorm hits. A drenching rain! and lightning! After the storm passes though, - Shh- the parks are lovely after dark. (I suspect many of the folks saying the weather is miserable made the mistake of toughing out the afternoons.) Be sure to bring water-tolerant shoes! Oh, and avoid those misting fans, they just = clammy. Dry/wicking and hydrated is the way to go. As a runner, I'm sure you know electrolytes are important.
Oh- one more important detail- wearing a mask in the high heat (and rain!) does add an extra layer of challenge. Masks in in temperate weather are less of an inconvenience IMO - if WDW still requires them come summer.
Your best bet is to consult actual weather average tables to see the numbers.
