Actually studies have shown that anything over 3 miles do very little for an athlete and actually can cause more harm than good. In sports such as TKD were you need short burst of speed rather than marathon like stamina sprints, stairs and those type of activities are going to give you the most benefit were long distance will not. Fit the activity to the sport to get the most benefit.
I agree. Running is ok in the initial training stages to help get you in shape, but I think running over 2-3 miles builds the wrong muscle fibers. Those with high levels of fast twitch fibers will balk at the distance running and will cause them to hate and resent training. You don't make a throughbred pull a plow all day. Better to do sprinting, stairs, bounding, rubber band, or other types of drills, which creates speed instead. I think their time is better spent doing specialized weight training instead of distance running, which makes your body, mind and spirit slow and methodical.