Pike's Peak Marathon takes place every August and is one of the most grueling races in the world. The race starts at 6, 280 feet and climbs a staggering 7,830 vertical feet to reach a summit of 14,100 feet before a bone-battering descent. This race makes ordinary hill work look like anthill work. You also have to guess that the vast majority of the competitors are ill-prepared for the extra demands of vertical mileage, let alone the impact of altitude. The average ascent takes runners four to four and a half hours, for a total average marathon of six and a half to seven hours.
Running at high altitudes decreases the amount of oxygen getting to the muscles. A low atmospheric pressure in the thin air makes the blood less oxygen-rich as it travels to the muscles. As the marathon proceeds and runners climb higher, the problem gets worse and worse as the runners' oxygen demands increase. Regardless of whether a runner lives and trains at a high altitude or not, high altitude slows performance.