All the talk in this thread about training and power output is great, but, it does not do any good if you do not eat enough of the right food (what food is somewhat dependant on the individual and what they can stomach, and the amount is a lot more than you are used to eating I would bet), if you don't give your body enough energy to perform and then rebuild you will not be able to push your limit as far, as quickly.
Plus recovery exercises and long walks/ runs after a hard training effort, as well as some proper down time to get fresh legs again, will make as much or more difference in how you feel on the bike as the training does. If you do not start the group rides with fresh-ish legs then that is another problem...

Fuel. Train. Recover. Fuel. Train. Recover...

check your fit and eat a steak!