Once you can ride a decent bit, you can typically ride a whole lot more if you take care of your exertion levels and nutrition. That you can't hang with a group over a particular distance is an issue of your aerobic fitness rather than your endurance.

I'd do your rides on whatever day you have your group rides. Keep doing the group rides and you'll likely get better. On another day you could try some 5-20 minute intervals at various intensities and work on improving across the board.

Also take notice of when and where you're getting dropped. Can you ride more efficiently (out of the wind, closer to wheels, etc)? Can you manage your efforts (less surging, more gradual accelerations, staying seated, starting difficult hills at the front rather than the back) or nutrition better?