I'm a recreational distance runner (specializing in marathon) who started doing some cycling for fun a couple years ago. Your main problem starting out will be to make sure your bike fits properly and then literally getting your butt in riding shape. If your fit is good, then you're less likely to get some repetitive motion injury (like knee pain from too low a saddle). If you haven't been riding, it will take your butt some time to toughen up to be used to sitting on a saddle for hours on end. It's a lot easier to go from running to cycling than vice versa; I rode a century and had fun just 2 months after getting my new bike.

Group rides are fun, just make sure to watch the group carefully and learn the etiquette and signals the group you ride with use. The main thing is to ride predictable, no sudden moves.

I just googled "group ride etiquette" and the first 3 links that showed up look decent:
http://www.oklahomabicyclesociety.co.../groupride.htm
http://bicycletrip.com/articles/grou...ette-pg150.htm
http://dailypeloton.com/gstepan.asp

Enjoy!