Not much to add that hasn't been said. As a Floridian...it's tough this time of year. Today I started my run at 6:30 (knew I would be out there 2 1/2 to 3 hours). I felt better today because I started earlier AND I think the humidity was a smidge better. (I know 6:30 is not super early, but it is my day off after working all weekend!) My goal was to be able to do my long runs w/ an 11 min mile avg in this heat. (I can do 10 min miles when cooler). I know the heat slows me terribly and if you don't respect it...it will get you. I must admit I think the heat & humidity started earlier here this year. And I don't think I handle it as well as when I was a few years younger. It has really gotten to me last couple of years.
Occassionally I see someone running at 12 noon or even 2 or 3, it is SO hot and the sun is so strong. I really think those people are crazy or fools for not being smart enough to run at more appropriate times of the day. Then again, maybe it is their only time to run or the heat doesn't kill them.
Did you see the article in Runner's World about drinking an icy beverage before run to cool your core. My problem is I need my coffee in the AM!

K