We won't be doing 40 miles this year; several of us were training for long rides 2 years ago, so that's what we did. Don't worry, you will be fine. I would estimate any ride we do will be around 25 miles. If we do the hilly route, naturally, speeds will go down.
I'm a ride leader with AMC and will review all the "rules" so you will feel comfortable. And, we will wait and regroup for anyone if need be. Last year I had a friend come as a sweep, so no one felt uncomfortable.