Olympic walkers are averaging more than 10 mph (less than 6 min/mi). Most fit people cannot RUN a mile that fast. So, 6 mph may be fast for a normal person, but it's well below the bragging rights pace for the sport of walking.

For me, 6 mph (10 min/mi) is a light jogging pace. A max walk speed for me is somewhere around 4.5 mi/hr. I could walk at 5 mph for a short burst. I don't think it's possible for me to walk at 10 mph even for a short burst, and I'm not interested in risking an injury to try!