Holy mackerel, you sound so much like my neighbor/friend that it's scary! She's about the same size as you, maybe an inch taller, also all legs, and she just got a Gary Fisher Nirvana!!! She'd been riding a heavy Trek mtb that she'd gotten with credit card points, and after just a few weeks traded that in for the Nirvana. Still kinda heavy, but miles ahead of the mtb. We ride together maybe once or twice a week (she's a flight attendant and I'm a teacher, so our skeds don't always match up), and I'm working on her to do a 42-mile ride with me in October. I was going to do the metric century, but if she wants to go, I'll do the 42 so we can ride together.

Ditto what Jenxxs said--you'll find there's a wide range of bikes at a century ride, and most likely you'll find someone who's riding about your pace. You may even meet a riding buddy. Don't make the mistake of wearing yourself out in the beginning by trying to keep up with the speedsters. Just set a pace you know you can do and enjoy the ride. Oh, and take full advantage of food and drink stops! Have fun and report back to us afterwards so we can give you as many "Brava"s as you can stand.