I’m sure you want to spend as little time in a car as possible, but if you’re willing to drive, the town of Fruita is 3 hours west of Breckenridge, and Moab, Utah is a little farther than that. It’s a pretty drive, at least. Moab has probably the most famous trail system in the United States, and the riding out of Fruita is very similar, and less crowded. Of course, if you’re traveling internationally to race, you may have already been there. 🙂
Aspen and Crested Butte are also in your range, and though I have no experience riding there, they are beautiful towns worth visiting for the scenery alone, and I have no doubt the many local bike shops would be happy to make recommendations.