The first thing to remember is that RAGBRAI is a ride and not a race. I agree with Zinger, don’t think OMG, I have to ride 80 miles today, think of it as 7 miles to town A, followed by 12 miles to town B, so on and so forth. Engage those around you in conversation. Everyone like you is taking the week off to enjoy the ride. Ask about their favorite food vendors along the route, things to see and do, but above all train. Ride, ride and ride some more.
RAGBRAI will post a list of items you should pack in order to have a successful week. You may want to consider packing light. 3 or 4 pairs of bicycling shorts instead of 7 (you can rinse them out daily while showering). Zip lock bags to protect your camera and cell phone (in case of rain) Pack your sunscreen, sunglasses, etc. and be ready for the exhilaration you’ll have when you dip your front tire in the Mississippi river once you’ve finished!
Enjoy, have fun and be safe.