Last winter, Jessie Diggins was just another up-and-coming American cross-country skier. At U.S. Nationals, she raced to a solid-but-unspectacular ninth place in the classic sprint, and at the 2010 World Junior Championships in Germany, her top finish was a 25th place, in the 10 k pursuit. What a difference a year makes. After collecting her first national title earlier this month, and only a couple of days removed from illness, Diggins placed seventh in the...