I think at some point we have all been dropped on a ride and it ALWAYS SUCKS. Nothing zaps my energy more than trying to play catch-up the whole ride. You are probably putting so much pressure on yourself because of this upcoming ride that you are actually wasting a lot of your energy trying to keep up with the group and worrying if you are going to be ready or not.

From the mileage you have talked about doing so far, I think you will be plenty prepared. Try to just relax and think of what comes after the MS150, what cycling things can you do JUST FOR FUN without the pressure of an upcoming event. You might try doing some training rides alone so you can hone in on your exertion levels, find a good pace and focus on your refueling. I find cycling alone makes me stronger when I do ride in a group.