Don't know if you or someone has already mentioned this, so I apologize. I have insulated saddle bags, which I highly recommend. They keep everything ice cold.
I would pack a ton of water, of course, and some ice packs. The best way to cool off in a pinch is apply something very cold to these areas:
Your wrists, the crooks of your arms, and your armpits. These are the locations of major blood flow that is close to the skin, and putting something cold here cools the blood very quickly. You can feel yourself cool, literally, within seconds.
Just reading this thread made me cringe - I hate intense heat. I went to Las Vegas one summer and was pretty sure I was going to die. Thank God I live in Indiana!
We drink to our youth, to the days come and gone; for the age of oppression is now nearly done.