My interpretation is that GP is observing that the benefits do not scale linearly. Most of the exercise benefit happens after a relatively short walk. The extra two hours a day for a long walk is expensive in time, but doesn't multiply the benefits 3x.
Walking is nice in that it's low-impact, although it's probably much more time-efficient to do a shorter, higher intensity exercise like running. (Saying this as someone who primarily walks and never runs)
