Scientific American answers Why Winter Endurance Athletes Compete In So Many Races "Cross-country skiers often collapse due to exhaustion at each race's end, only to come back a day or two later and compete again at seemingly superhuman levels. Speed skaters racing on the traditional large oval maintain a painful crouch at speeds greater than 30 mph in race after race throughout the games. How do they do it? And why don't elite runners run in as many distance events at the Summer Olympics?
Sports scientists say that the endurance sports of the winter games are more like bicycling or swimming than running. There just isn't as much pounding on the joints or the muscles, for example. Therefore, athletes can recover faster, and compete in more and longer races in the Winter Olympics than in the summer games."