Indysteel is right. The best way to get ready to ride your bike is to ride your bike.
Even though RAGBRAI doesn't have any century days, using a generic Century training plan would be a good way to ensure you are comfortable on the bike and at the end of each day and the whole ride. Build up progressively, generally 10% a week. It's especially important to do like Indysteel suggests and get in back-to-back long rides so your body is used to riding long and getting up the next day and doing it again.