I think it has to do with training as well. Addie (who is a year old) and I walk around the neighborhood twice, sometimes thrice. When we first starting walking, though, she couldn't walk one time all the way around. I had to pick her up at about 3/4 of the way. Now she can easily walk two rounds and often pulls for a third. Sometimes she can make the third, and sometimes she poops out at the end, and I have to carry her.
I do always take a look at her paws after a walk, though, to make sure the ground isn't rubbing her paws raw.