A learning experience. I bet your adrenaline was going pretty good since it was your first group ride so I'm guessing you burned yourself out on that first hill. Next time pace yourself a little better; or select rides that are not hilly or quite as long. The other thing is that your bike is not well suited for the ride you took, but it could be. If you're going to ride mainly on pavement, get a more road friendly tire (28's with a good smooth tread). Finally, the more you ride the easier it gets so keep going. Good story.