Wow, Hub--sounds exciting! Your first big ride!

I'm no expert, but I've done a few of these rides before, and here's my take: Have plenty of water and some kind of sports drink with you, plus some snacks that you like, but also take advantage of ALL food and drink stops.
Ride your own pace, or maybe just a tiny bit faster. Don't get caught up in the excitement of the group start and try to keep up with the speedsters at the front--you'll just wear yourself out. Set a pace that you know you can stick with. The adrenalin starts pumping in a ride like this, and it might get you through the 50 miles, but not if you trounce yourself at the beginning.
I'd say ride the bike you've been used to riding. They say never to test out any new equipment the very day of a big ride, and I think that warning includes trying out unfamiliar bikes.
I'd also say, if you have time, try using some old-fashioned toeclips. I use them and love them. Might make the big switch to clipless in a month or two, but toeclips have their advantages. Either way--toeclips or clipless--they help you to spin, which will help you get up those hills without killing your knees or your endurance, rather than trying to push big gears and hurting yourself.
Carry a spare tube, patch kit, and equipment to change a flat--maybe BBB can help with that.
Maybe some more experienced riders will chime in.
Whatever you do, HAVE FUN!