I'm doing my first Century all by myself this Saturday...so I'll be thinking about you doing your ride on Sunday!
I'm sure you'll be just fine by yourself. Last fall I did my first organized ride (Tour de Tahoe) all by myself...I had no idea what to expect and I was kinda scared...but it turned out great. Then this Spring I did a metric with a friend and I think I actually would've enjoyed it more had I been alone. Then you don't have anyone else's expectations to live up to, you can go at your own pace, stop when you need to, etc.
Good luck (to both of us!). Have a blast


p.s. I'm packing gels, Shot Blocks, a Power Bar, cashews, and extra Gatorade powder just in case I don't like what they have at the rest stops