I second what others have said about average speeds on group rides - it's probably in ideal conditions of flat road and no wind.

One month ago I was averaging 10-12 into the wind (out here it's usually 18mph+) and 14-16 on flat roads with a 21-23 with a tailwind. I ride about 120 miles a week and yesterday when I rode, I was averaging 17-19 on the flats, 28 with a tailwind and 13-16 into the headwind in average conditions for where I live.

I've heard others say the best way to get faster and stronger is to Just Ride Your Bike.