Hey, congrats! And, my .02 includes:

Re gel shorts mentioned earlier in the thread...as with anything, make sure you try out everything on a long ride (or several) before using them on a century. Preferences are so personal...I HATE gel shorts!

About the heat, there are lots of tips in various places around TE, so won't take too much space, but after a 6-day, 100+ degree tour a couple years ago, I totally second carrying electrolyte tabs (I used Nuun) in addition to your drink, and carry at least a bandana you can wet down and wear around your neck for a swamp cooler. You might also want to look into some products made specifically for that purpose that really hold water (and ice). Some motorcycle gear catalogs have some pretty neat items for beating heat, since those guys have to stay cool while wearing leathers, etc.

Have fun!