I just finished a 1000 mile ride from KCMO to Rome, GA. Man did it get freaking hot and humid! Temps in the upper 90s and humidity at above 80%. One thing I learned several years ago is to take a long sleeved cotton shirt and soak it. Put it on. I really does wonders for the heat on your back and arms as you ride (providing you have extra water!)
I also stuff a bandana under the back side of my helmet and drape it over my neck to help keep the heat and sun off my neck. Seemed to help.
I've heard awesome things about using a Camelbak. I haven't tried it but everyone I know that rides with one swears by them.
I did ride early morning, goofed off during the hottest part of the day, and resumed later until about 8:00pm. Then looked for a place to pitch the tent. I averaged about 60-70 miles each day with a couple of lesser days and a couple of 80+ mile days.
All good tips above.
Jerry H