Well, the average American is taking less than 5500 steps a day. 2000 steps average for a mile, 2.75 miles average a day for your typical redneck ( )
The average Amish is taking 18 000 steps a day, 9 miles. Strangely enough, they almost have no weight problems, few type 2 diabetes and other weight related illnesses.
But you could always argue that 9 miles are somewhere 2.5 to 3 hours a day. A lot of time in a society organised around cars, with great distances to be covered for your work, distances that you can not walk/bike conveniently, let alone cover with public transportation.

I do not mean to say this is all society's fault, but it plays a great part.

Now if someone tells me 15 miles is easy for the average person... well, it's 66% more than your average amish, so I'd say bullshit
And remember that it's 5 hours for an average guy that walks and should still be able to hold a conversation breathing normally