I'm right there with you. I'll be doing a metric tomorrow. I live in Maryland. 90/90 days are common here.
I'm assuming that your ride will start in the morning (I'll be off by 8). 50 miles won't take you more than 4 hours, especially on level ground, so you should be done by the time it gets really hot.
As Sarah said, use every rest stop and refill your bottle(s) each time. Drink before you are thirsty. I don't know that I'd mess with a camelback unless you've used one before. Rule #1: don't do anything new on your longest ride.
Obviously, wear the lightest (material and color) jersey you can. I like REBCCA's idea of a camisole; I'm going to see if I can find one in a men's large for tomorrow.
Shade where availabe will help. The evaporative cooling from riding will also help. The wet kerchief is a good idea as is soaking your jersey (hint: if your jersey pockets are empty, a few ice cubes help).
Relax ... enjoy the ride.
I'd rather be sitting on my bike thinking about God than sitting in church thinking about my bike ...