I do most of my riding on Colorado's front range in an elevation band that ranges from 5 to 8.5k ft. above sea level. Truth be told, I notice very little pain difference between climbing here and up on the high passes, like Loveland (at 12k). I probably, instinctively, just adjust the pace. Two weeks ago in the Triple Bypass, I met folks that came out from sea level or thereabouts and acquitted themselves quite well.