rms,
Sounds like your doing things right. first recommendation, ride with a group until you keep up with them, consider this a harder ride. Second recommendation, do solo rides at an easy pace to continue to build up endurance. Find a balance between fast group rides and easy long endurance rides. Try to ride 4-8 hours per week balancing between easy rides and harder ones.
What is interesting is in group rides is that you either work really hard or almost not at all, but always go way faster then being solo. The key will be is not letting a significant gap open up with the rider in front of you. This is especially important if you are the last in the group. Once that gap opens, you may never close it again and be riding home solo. Be sure you have a map or good idea on how to get home. Group rides often force you to go 100% to stay attached, but reward you with a nice easy coast if you stay on with the group. Until the next acceleration, so you do it all over again.