There are a fair number of winter “ultra” races where the race course is “ultra” in its length. In the Mountain West there is the Fat Pursuit; in Alaska it is the Susitna 100 and the Iditarod Trail Invitational (which follows the sled dog route all the way to Nome.)
The Midwest also has a big, long winter race: the Arrowhead 135. Like most winter races, it's open to bikers, runners and skiers. It starts in International Falls, Minn., and ends 135 miles later near Tower, Minn. It's held in late January. Every once in a while it's in the 20s or 30s, but is usually quite chilly — sometimes lower than 40-below zero.