Matt Liebsch couldn’t believe the nerves he was fighting before the 2012 American Birkebeiner. A Minnesota ski hero at the ultimate of U.S. cross-country marathons, Liebsch had won the 50-kilometer skate race in 2009. In early February, he took back-to-back victories at the 2012 Boulder Mountain Tour and City of Lakes Loppet. But the last time Team Birkie’s lone elite skier raced the Birkie in 2011, he was ninth. And he hadn’t competed in three...