Not sure how 6 miles per hour can be construed as walking speed for the vast majority of people. At the gym and on the treadmill, at 5 to 6 miles per hour I am in a light to moderate jog.

With walking the urban concrete jungle or hiking trail, there are far too many variables to take into account such as (but not limited to): terrain, elevation gain/loss, physical and physiological condition of person, weather (heat/cold/humidity/rain/snow), weight of pack, type of footwear and so on...

The linked chart that Yelp provided is far more realistic keeping in mind that that is based on overall and generalized averages. YMMV (Your Mileage May Vary...and no pun intended.)