Basically a marathon but on snow with planks on your feet, and yes, people do this for fun.
Fun fact: Cross-country skiers consistently record the highest VO2 max values of any athletes in any sport.
Just swimming, cycling, and running back to back for people who decided one sport wasn't quite enough punishment.
Fun fact: The Ironman was born from a 1978 bar debate in Hawaii over which athletes were the fittest.