If your immediate goal is to improve endurance (in preparation for the MS ride), I would try to get in as many centuries or long (80+) rides as you can before the event. Then, if possible, I would ride a little over half the distance of the previous day's long (80+) ride. For example, try to do 80 miles on a saturday and then 45-50 on sunday. This should mimic the type of exertion, recovery, exertion that you will be experiencing for the MS ride. Sadly, there aren't very many weekly group rides that are centuries. Even if there were group rides specializing in long distance riding, it would be hard to find a group of riders that are similar enough to maintain the same pace over a century ride. I suggest you train alone. It would be the best way to isolate what you want to work on, when you want to do it. If your goal is to just gain speed, especially over shorter distances, then I suggest you go to some of the faster rides and just make it your goal to go farther before getting dropped than you did last week. Eventually, you will stick to the group for the whole ride. The motivation of not getting dropped is a tremendous motivator and, in my opinion, the quickest way to push yourself and become a faster rider.
Good Luck.