Altitude acclimatization can be different for different people, but for the majority it's no big deal. We live near sea level and last year my wife and I flew to Denver on a Sat, did a 110 mile ride with 12,000' of climbing the next day, and then the following week did a 500 mile multi day trip through the Rockies that included 28,000+ of climbing. We experienced no ill effects other than undoubtedly going a bit slower than we would have a lower altitude. There were a dozen others on the 500 mile ride, all from lower elevations, and they all did fine too.