Im not sure anyone has said it on here or if i just missed it. Ride some hills, get used to shifting on hills. If you dont have hills in your area, search some out or ride into strong headwinds (to simulate hills). Find an overpass and ride it all day say 50 times in a row. You may have days like this on RAGBRAI, this is supposed to be a flatter year but dont be suprised if there are rollers all day later in the week. I rode the whole thing the first year I went (3 years ago) and I was fine, in fact I did get stronger as the week went on. I think i had just over 1000 miles in me that year before we left for RAGBRAI. I had a couple weeks where I rode 3 or 4 days in a row leading up to it, I had a couple 50 mile rides and a metric century (62 miles) before RAGBRAI that year. I trained on hills though so I did fine.