Would like to try RAGBRAI, have not had the pleasure though.
As for the post above mine, doing these sorts of rides it is imperative to get out at the crack of dawn in most locales. All the bad stuff happens in the afternoon - heat, humidity, thunderstorms, etc. When I did Bicycle Tour of Colorado a few years back, I was up at 5am for breakfast, tent packed up by 6 and on the road at 6:30. There are thunderstorms on the passes almost every afternoon and you want to be over them early in the day. And at the lower elevations it gets hot in the afternoons. I always tried to shut it down for the day no later than noon, and preferably by 11am. Your body adjusts pretty fast to going to bed at dusk and getting up at 5.
The only day that I pushed later into the afternoon on said BTC tour, I was of course hit with a thunderstorm on the last of three 10,000+ foot passes of the day. Pretty scary but not much to do other than push on as there was no shelter. 102 miles from Creede, CO to Gunnison, CO with a lot of climbing and thin air, and it was only the 2nd day of the tour, by the last day I was better adjusted to the elevation. I think on that 2nd day I arrived around 4PM. All the other 6 days, I never arrived later than 1.