They say that 8 miles per hour is the most efficient speed for the average man to run at per mile. That is, shuffling a 6 mile per hour pace may be lower impact, but opening up your stride a bit saves steps leading to energy saving. So at an ideal pace, you take off running. How far could you get? You are of course allowed to pace yourself and start out at 6 then negative split up to 9 mph, but this is a question of aerobic endurance, not pure durability so the run has to average 7.9 or more miles per hour to be valid.