Some fine advice above. I'll chip in my own 2c:
* It is a courtesy to let a group ride leader know in advance if you are thinking about not returning to the ride start. In my club we make sure everybody gets in safely and somebody would end up backtracking looking for a missing rider to make sure they were OK.
* Don't be intimidated by fancy new bikes and equipment. A bike isn't fast without a good motor turning the pedals. Appearances can be misleading. Last night I was feeling smug about [almost] keeping up with a very fit rider until I found he had just donated 2 pints of blood.
* Group riding is valuable experience and makes it automatic to signal/communicate your intentions, hold a predictable line, and be more comfortable in a crowd.
* 13 mph is the average speed of many of the cyclists in my bike club who ride about 100-250 miles/month. You may find a club in your area that has rides for people who are not on the LBS' bike racing team nor rookies who just bought a bike.
* Sometimes I enjoy group rides with faster riders especially on out+back routes. I chase after them as hard as I can. I turn around early and see how far I can go before I get passed, then I chase after them hoping to get back to the ride start in time to chat. It's a good reality check. Watch how they ride and see if they are doing something differently than you do that would make sense to try.