I hail from FL. This time of year is about survival until the first cool front comes down sometime in the mid to late fall (Halloween to Thanksgiving). As others have stated:
1) Forget about your winter paces. You're gonna run 30-60 seconds or more per mile slower. It's life. The best way to know your workout paces is to run a summer 5K or TT and use that.
2) Hydrate and eat! You can have more salt in your diet since electrolytes will be lost.
3) Rest. This is both sleep and cutback days and weeks. I had to make this week a cutback due to the same depletion you're feeling.
4) Avoid the sun by running when the sun is down or finding shade.
5) Avoid asphalt or concrete as these radiate heat.
6) When pace/distance is king for a given run, it may be necessary to move that indoors if the dew point is too high.
7) Ice and cold fluids do wonders at reducing core temps on hot days. I've been made fun of for stuffing ice cubes into the pockets of my shorts and carrying ice cubes in my hands in races, but it does help!
Others on here have commented that day by day accumulation of this heat stress takes a toll, and the resultant depletion has a lot to due with you sweating out your electrolytes and nutrients. It's important to recognize that and take cutback days/weeks or move some runs indoors to allow your body a chance to replenish. I've heard of some creative runners even using pool running for relief from time to time.