Ages ago, in another life, I was a runner. While on the Cross Country team at Oxford, I trained an average of 10 miles per day. No longer. After a few years of subscribing to Medicine & Science in Sports & Exercise, I wised up to the superiority of High Intensity Intermittent Training. Heavy squats done in a Tabata protocol probably do more for your heart than running 10000 meters at a world record pace. And the squats take just a fraction as much time.