If a usual marathon isn't extreme enough, you definitely should try this variation in Greenland. It's called the Polar Circle Marathon, and it's far more demanding and challenging than a usual marathon because of the cold temperatures and the slippery surfaces.
It's also probably one of the most spectacular marathons in the world with the route going through tundra, moraine landscapes, glaciers, Arctic deserts, and with some parts even going over the actual ice cap. If you're lucky, and the weather is beautiful and sunny, the scenery will look even better with sparkling snow and frozen blue lakes and glaciers. Don't forget to bring sunglasses and warm thermal clothes. The Polar Circle Marathon is held annually in October in Kangerlussuaq, West Greenland.