Part of the challenge of the Leadville 100 is the altitude. While the advantage clearly goes to those who live at high altitudes, or choose to sleep in an altitude tent leading up the event, those who live at lower elevations can take steps to prepare for the thin air.
It takes months to truly acclimatize to elevations such as Leadville, but two weeks tends to be an adequate amount of time for the body to start making physiological changes to adapt to the lower oxygen levels.
In an ideal world, it would be best to slowly gain altitude and acclimatize in stages, much like high-altitude mountaineers do with different base camps that they ascend to for training and then descend for recovery. Unfortunately, this luxury is not available to most, but there are other options as well.
Altitude sickness generally takes some time to set in, so those coming from lower elevations can attempt to go up to altitude as close to the race start as possible and then return to lower elevations as soon as possible after the race. Hydration is also crucial and can be aided with a vitamin mix such as Acclimate, which can be consumed before arriving at altitude and while there.