Their fitness levels adapt, all you get when you walk dogs such long distances evry day is very, very fit dogs, it's hard to keep that up so then you get bored dogs with lots of energy they can't expend. Great!

I do 3 walks a day 2 x half an hour in the morning and before bed, one longer one at 6. I try and get out of the city and do a few country/suburban walks at the weekend.

Worth noting that wild/street dogs spend most of their time lounging about and hardly run at all unless necessary...