The leash goes taut, and suddenly you are cargo. That is the moment when most newcomers to skijoring understand the bargain. One second you are a cross-country skier, in charge of your own pace and direction. The next, a dog has decided that the trail is hers, and you, all sixty or eighty or a hundred and seventy pounds of you strapped into a hip belt and a bungee line, are merely the most interesting...