I'm also bigger than average, and after a year off from cycling, my first few rides were in the 13-14 mph range (on the lovely rolling hills of SE Michigan). If you stick with it, your speed will improve. And even though it can be intimidating, riding with faster people will help you get faster too. Others above have advised you to contact the ride leader, and I second that advice. He or she will give you an honest assessment of whether or not you can keep up with the group pace; and if not, hopefully s/he will be able to point you in the direction of a different ride that is more suitable. Ultimately, it's more important that you enjoy your ride than that you feel badly for not being as fast as everybody else (this is what I tell myself every day!).