I left from Atlanta (1000 feet) and dropped into Frisco, CO (9,000) and did rides that maxed out at well over 11,000 feet. I did all the usual precautions - lots of carbs, LOTS of water, avoid caffeine and alcohol, and take it easy the first few days, getting plenty of sleep, but if possible, do a short workout at altitude if you can allow enough time to recover before the Big Event.