There's a middle-aged man you can spot most days at sundown on the north beach of Lake Harriet in Minneapolis. Wearing just his swim trunks, he wades into bone-chilling waters, calmly breathing and smiling, until his toes can't touch the ground. Then he swims. He is the furthest thing from a triathlete in training. His technique looks like a cross between a front crawl and a doggie paddle. But ...