it depends on what you define as a "run." I "run" at about 6 mph or 10:00 min/mile. Most "runners" I know have done cross country or track previously (I haven't) and they say faster than 7.5 mph is "running." I call my outings "jogs" so as to not offend the running purists who say anything slower than a sprint is just walking.  I can "jog" at 6 mph for 6 miles consecutively, but if I'm "running" any faster than that, I can only make about 2 miles before my legs hurt. I don't like the super high impact associated with running fast, though, so I stick to my "jogs" of 6 mph. Whatever works for you and makes you happy :-)