You'll do fine! You're in way better shape than I was, or am. I'd also never done more than 25+ miles until the day I did 42 in strong headwinds on a comfort bike. They say you can do at least twice as much on an organized ride as you can do on your own, unsupported. As the others say, you may be sore here and there after from being "in the saddle" longer than you're used to, but you'll have fun and you won't mind the soreness, which you'll recover from anyway after a few hours. That's my prediction, at any rate.