You should definitely check with the group. Here in L.A. we have a great club called the L.A. Wheelmen, which always organizes three rides, short, medium and long. Those three groups usually naturally break down as slow, medium as fast. They rarely drop folks.
Every club has its own goals. You can probably find a group in your area that will fit yours.
One other note: I'm in the same boat. I'm a clydesdale that is trying to go from 255 lbs down to my old riding weight of 180 lbs. So far, 10 down, 65 to go. I'm pretty strong in the flats, but so damn slow on the hills that I've told myself I'd give it a year to start riding big group rides again.