Because of the nature of the run (dirt trails, rock, and other natural obstacles) and the high altitude, the race is much more difficult than standard 42.195-kilometer (26.219 mi) marathons. Winning times for the marathon are typically just under four hours (compared to elite "flatland" marathon times of just over two hours). Although the average grade of the slope is 11%, some sections are much steeper because the central portion of the race is relatively flat. The initial three miles (5 km) are very steep. The central 7 miles (11 km) start as rolling terrain, but become progressively steeper toward the end. The top 3 miles (4.8 km) are above timberline and require some rock scrambling to reach the summit. Oxygen levels drop progressively as altitude rises, further compounding the uphill ordeal.