Ditto on the above! Relax and have fun. I love the quote that HoosierGiant posted, it's good advice to keep in mind.

Back when I used to ride a lot, I did long, organized rides all the time. I realized that I usually finished near the back of the pack (with the old folks and little kids ) but I found that it didn't bother me because I just really enjoyed being out there on my bike.

You can do it!