On Saturday morning, I will be just one of the nearly 15,000 people expected to fill the stadium and line the trails for this weekend’s World Cup races in the ski-crazed nation of Estonia. The country of 1.3 million—the same population as the state of Maine—has produced six Olympic medals in cross-country skiing over the last decade, courtesy of three legends: Kristina Smigun-Vaehi, Jaak Mae, and Andrus Veerpalu. Just how serious about cross-country skiing is...