I take it that you are doing a charity ride like an MS 150. Been there, done that. I was in the same boat you are when I did my first one and I was worried about finishing. It actually went a lot better than I thought and before I knew it, I was going across the finish line for the first's days ride. What helps is to find a rider that is on your level of riding and turn him into your riding buddy. The conversation takes your mind off the ride and makes it more enjoyable. Two of my team mates became my riding buddies for the first ride I did. I also listened to music the entire time, but you can't do it with headphones or earbuds as they are not allowed on these rides. Anyway, I think that you will do well. Just keep riding and don't think about the miles you need to do and "Just Do It."