Could marijuana be the key to getting more Americans on their feet and exercising? The stereotype of the "lazy stoner" is ingrained in our public consciousness, but a new book tells a different story: about the amateur and elite athletes who are using cannabis for the purpose of exercise. In Runner's High, which is out now, investigative journalist Josiah Hesse explores this subculture, outlining how the practice could fundamentally alter our perceptions about exercise and…