If you're already doing 25 to 30+ mile rides now, then the 40 miles shouldn't be a problem IMO. You have four weeks to try out the clipless pedals. If you still don't feel confident riding with them by the time the ride comes around, you can always change them back out for the ride.

My first experience with an organized ride was great. I think most of the experienced riders could pick me out. They gave me space when passing and many even shouted out encouraging words, which was most appreciated on the hills. Cyclist in general are a cool lot. Most of them are out to have a good time and it's fun to see so many people out on bikes. I think that most of them will understand (with the exception of a few jerks, but don't mind them), they were all noobies at some point. Definitely take the advice of others and try to find some organized training rides (sometimes put out by the group organizing the ride) in you area or some riding buddies. It'll help build up your confidence around other cyclist and you might be able to find people to do the ride with which would make your experience even better.

Good luck and have fun on your ride!

Soo