I agree with Lauren, I think if you are going to be doing long distances, clipless pedals would be the way to go. I had the same experiences where the LBS put me on the trainer and I clipped in and out until I was comfortable. Although I wasn't as skilled or lucky as Lauren in that I did fall eventually, it wasn't bad and my partner looked at me and said "Congratulations, now you are a real cyclist!" There is a lot of info on pedals in the threads here, good luck on that and on your big ride.