Nordic skiing has been mentioned a few times. I race on skis, bikes and foot and all top athletes are equally impressive. Cross country skiers stand out since they not only need endurance and speed, but they need to do a lot of work on power and technique. Coordinating the power and adjusting technique to conditions in xc skiing requires a supreme athlete with top balance and coordination (genetic and trained). If you have ever tried xc skiing you know that fitness is just one part of the equation. There is a high degree of coordinating the arm and leg fitness that takes subtle and powerful athleticism.
A friend of mine graduated as one of the top 20 NCAA xc skiers in the USA. Not quite US Ski Team material, and no career skiing opportunities exist, so he took a job in Boston and stopped skiing. He started running seriously that summer and raced the Boston Marathon the following spring. I was blown away by his 2:24 time. That is not a lot of running training and a testimony to his base fitness. Ben True also comes to mind as a top NCAA skier who chose to run over ski only because he could make a living running. Of course, a top 20 NCAA xc runner could train for the marathon and run 5 to 10 minutes faster, but these xc guys are fit and amazingly coordinated athletes.
Skiing, unlike running, does not typically cause overuse injuries, so these guys can and do put in mega hours on skis. They can spend 3-4 hours on skis average per day.
I would bet that most top xc sprinters would be unbelievable decathletes. Keep in mind that XC sprints last 3 - 4 minutes, so these guys have miler engines, with quick twitch muscles and super strong upper bodies.