Ask the guy to send you a map of the route in question. Then go for the ride on your own, do as fast as you can, and see what your average is.
If you can complete the ride and your average is say 11mph, then a group ride of 12mph should be ok, you'll only be a little bit behind.
You can always tell him you will go on the first half of the ride this time and turn back on your own and head home half way through, if they will allow that. That might be a nice way to break into it easily.
And yep- 20-25 miles is a pretty small ride for a typical group ride. Groups don't usually bother doing 10 mile rides because lots of people drive to the ride starting point and it just isn't worth the hassle unless you can ride for 2 hours or more.

Good for you Donna, getting into all this!!!