And your friends probably meant 15 mph average, not the speed on the flat, which is a whole different thing. 16 mph average is decent for a hilly course, the fast folks might make 18. So trying to do 15 as a beginning rider is tough. You done good. Don't be afraid of riding with them again. After a couple of months you won't be getting dropped as much, or at least not as quickly. A great way of training to get faster is simply trying to hang with faster riders.

Depending on where you live, you might want to ride outside all winter, at least once/week. It's not nearly as bad as people think as long as it's not icy. It's just a matter of having the right clothing.