Here's the worse that can happen... you start out with them, can't keep up and then stop, tell the guide you want to return to the park and you turn around.

I have to tell you I feel even 13 - 15 mph for a "newbie" ride is very fast. Yes I know new cyclist can ride that fast but on many of our rides we have folks that can barely go over 5 - 8 mph (really I'm not making this up). And even 13 miles sustained for 25 miles for a new rider is alot.

The 14 - 15 mph pace for the shorter ride (your LBS' ride) seems very fast. I would make sure the leader/guide whoever is talking you into joinging agrees to stay with you no matter what even if you fall far behind the pace. If he cannot. I would hold off the group ride. Do some riding on your own, feel comfortable with the distance and pace and then give it a go. While group riding can be fun it is also difficult and requires some skill.

If the shop has a normal route, get a route slip and ride it by yourself to get comfortable and then tackle the group ride.