Mark Sisson shares his thoughts with Brad Kearns on the surging interest in "zone 2 cardio."

Most conventional fitness wisdom will tell you that you need to break up your week in a regimented manner, spending a specified amount of time in different heart rate zones. But how necessary is that really? Is our obsession with zones just the latest revival of chronic cardio? Hear what Mark and Brad have to say. #ListentotheSisson