hi all - a question. i'm considering a marathon swim of 20 miles next july, and believe I can put in the training to be successful at it (prob 11 hours in the water). My concern is this: i've lived the last few years at 200 feet elevation and the lake is at 6000 ft. I'm sure in the months as I train i can get some time in at higher elevations, but they would probably be no more than 2000 feet. How significant is this change in elevation and would it drastically affect my ability to swim for 20 miles, even if i have the endurance? I'll only have a couple days to acclimate to the elevation. Thanks.