The SD would have break in the middle just like we do, to run, stretch the legs and rest a bit and then back to work but otherwise, they are on duty the entire time.
Of course they have breaks. As do the dogs you are speaking about. What happens between the walks you are talking about? Down time. Same thing. Basically. Even throw in competition dogs. Same thing. And most live for the sport.
I am not advocating 15 hrs of walking a dog and seriously doubt anyone is walking a dog 15 hrs a day but what you are describing isn't much different then a service dog that works fulltime. I absolutely do not see the difference.
But since we're in a PTSD forum I was kinda implying we're talking >more< (but not exclusively) about psychiatric SDs
I'm talking about service dogs as a whole. But, don't forget that there are many mobility tasks possible with "just" PTSD. Such as foward momteum. Whether to use it as a guide when disocciating and guiding you out of a place when disocciated. That is a mobility task at it's core. Used for just PTSD. As an example.
I think many owners do need to teach impulse control and learn how to be the center of their dog's world.