I agree with ttaylor. Sounds to me like you are very prepared for the MS ride. It is not a race. It is a charity ride!

It is possible you didnt have the best ride with the best group that night but I wouldnt let one experience set the tone for the fantastic things you are doing.

Personally I like to ride with stronger riders on the weekly group ride because my riding partner is not as strong as me. I can push more when I ride solo but the group can really make me push that little bit more. I can really see and feel a difference afterward in my speed and endurance.