My sister's friend in Louisville did it a few years ago. He trained by filling a pack with as much weight as he could, then walking up and down the stairs in one of the downtown Louisville high-rise buildings for hours at a time.
Doing that sort of stuff isn't that hard physically, but mentally it's just awful. I did an hour and ten minutes on the stairmaster (the one with actual steps) today with a 55 lb pack. The clock seemed like it was going backwards.
I did the math - based on the number of steps I took at 8 inches per step, I'd get to the halfway point in about three hours (5,000 feet elevation gain). You rest up at that time, and then wake up at midnight to go to the summit (another 4,000 feet or so, but with a lighter pack). Then you go all the way down that same day.
So as long as I can extrapolate my 1 hour on a stairmaster at about 800 feet elevation in the comfort of a YMCA to going for two days at altitude, cold weather, crevasses and potential avalanches, I'll have nothing to worry about. Besides, you don't want to over-train!