Walks with Einstein start out "omigod, I'm so excited to go for a walk -- hurry Mama, let's see how fast we can walk to the next block" after 15 mintues they sound like "how much farther? are we there yet? wait, I have to stop and smell this, and this, and this, and this,"
So generally our walks last about 35-40 minutes. THAT way Mama can have a peaceful night's rest. Otherwise (like on rainy days we've had here in Houston lately) I feel like I'm getting up for 2 am feedings again (my youngest is 18).
If it was up to Einstein, we'd walk 15 minutes, stop and rest. Walk 15, stop and rest. I noticed he plays the same way when he's in a group of dogs, too. Play, rest. Play, rest.