Originally Posted by
brianogilvie
In my experience with exertion at altitude (backpacking, not cycling, in New Mexico, between 8000 and 11000 feet), allowing a day or so to acclimate to the elevation was sufficient. I certainly noticed that I was not as sprightly as at my normal 200 feet above sea level, but it wasn't debilitating, even climbing hills with a 40-lb. pack.
"not as sprightly" is pretty much my experience. I live at 5,000', and used to backpack the San Juan mountains in Southern Colorado at elevations from 7,000 to 12,300. 7,000 wasn't noticeable, while anything above 10,000 was indeed not as sprightly.
Even among the most fit persons though, there are rare but enormous differences between average tolerance and extreme sensitivity to altitude. 9,000 feet will probably only slow a flatlander down a little, but if you experience altitude sickness, call it off and head down.