Please bring your bike in ready-to-tour condition. Before facing a serious downhill such as the ~3,000 foot drop from Slumgullion Pass down into Lake City, I find it comforting to know my brakes and tires are in top notch condition. Nothing ruins a good day faster than a silly breakdown that should have been taken care of before tour.
If you are a flatlander, especially one from low altitudes, your body will complain from lack of oxygen. We strongly suggest you arrive a few days early and get in some high altitude riding to help your lungs and legs adjust. Wikipedia article on altitude sickness
Before you leave home: at a minimum, you should be able to ride two consecutive 60 mile days comfortably and be willing/able to ride a third day.
Bring clothing for every possible weather situation. It can and does snow in Colorado 12 months of the year. It’s not typical to see snow in the summer, but at the top of the passes, you might. Bring winter weight gear, including tights, long fingered gloves, shoe covers, etc. They will probably stay in the bottom of your bag – but if you need something, you’ll have it.
What you most likely will need is sunscreen and plenty of it. Get the high SPF kind. The sun is closer in the Rockies!