I definitely think you should do the ride, rivergrl!!! Shoot for the 50, and as the time gets closer, if you feel you can do the 75 then go for it. You can always change your mind the day of the event if you've chosen a longer distance, but don't feel you can go that far (some days you just wake up not "feeling it").
There are also lots of training programs online- just be careful of the source (don't do Bob's training plan if you don't know/can't trust who Bob is ). Since you can easily ride 21, the 50 might be a great challenge for your first group ride.

Like others said, it's never like the peloton that you see on t.v., and the rides are always far more relaxed, easy-going and friendly than any serious "races." You'll have the cyclists show up who have something to prove and want to speed through, and you'll have the recreational cyclist who's just there to do the distance and have a great day!
I'm doing a ride tomorrow, and athough I'm more of a serious cyclist, I'm taking my "more relaxed" road bike and planning on riding it with a couple friends and having a fun day. My alone training rides are where I can really kick it and race myself. Tomorrow's going to be a fun day!

Enjoy!