I agree with the comment about interval training. We live in Houston where there are no hills at all. We substitute with interval training and I have not walked a RAGBRAI hill yet. That includes twister hill last year and Potter Hill in 2010.
Also, this is the flattest course in a while, so I don’t think hills will be much of an issue.
Nevertheless, if you are inclined to walk, by all means do so. I guarantee you will have plenty of company on the right side of the road.