Some questions to ask the shop:
Is there a waiver you need to sign to join (danged lawyers!)?
Do they have a cue-sheet or map you can look at?
How many folks take part in the ride?
Is the ride broken into groups based on speed?
Is it a "no drop" ride that someone on a hybrid is welcome to join?
Is there someone riding sweep to be sure no one gets left behind?
Do they encourage newbies and work on group riding and skill building?
I agree - a 12 mile ride doesn't sound like it would be a race training ride, so I would imagine you'd do fine on it.
Good luck and let us know how it goes!