First, don't be embarrassed. I went with my local club and the pace was faster than advertised, but the sweeper guy and I had a good time. Toward the end the ride leader came back to be all encouraging and supportive and whatnot. Twenty years ago I would have been mortified to be the last rider, but now I just figured, well, I'm doing my best, if you don't want riders like me on your ride then don't advertise a 12-15 mph pace (I averaged 14.5 mph on the ride). Everyone was super friendly, even the people that I saw only briefly at the first stop.

So I guess my advice is to have fun and don't let it bother you if you are slower.