Three quarters of the way through Saturday’s U.S. national championship in the 50 k classic, when the men’s pack began the process of whittling itself down, an unfamiliar figure emerged from the mix into the lead group. Tucked into a sea of U.S. Ski Team and club uniforms, it was easy to miss the tiny form of Yuma Yoshida, who can’t stand much taller than five-and-a-half feet. But by the time the race wound down,...