Marathon Swim at 6000 feet

hi all - i'm considering a marathon swim of 20 miles next july. 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? My endurance is another question. I've completed one Ironman triathlon, meaning i've never swam more than 2.4 miles. I'm very comfortable in the water though and feel i can be disciplined over the next 10 months to get the training in. Anyone have thoughts on training tips in order to complete a 20 mile swim 10 months from now? Thanks much.