Ideas
Usually I find them just randomly stopped somewhere if they've been gone to long. I just never use long walk because it takes so long, I don't wait it out. When I do, they are still walking but for way more hours than is reasonable for a "sim day".
This is what I have been finding recently as well. sometimes (not all the time) I notice they have been gone a while and click the portrait to find them and the dog standing around in the middle of nowhere. I gave to send the dog home, then have the sim jog back. At least this still seems to count as having walked the dog. I have noticed it happening more in Chestnut Ridge (specifically Red Roan Field, in Riders' Glen) than in San Myshuno, (Old Salt House, in the Spice District specifically) or Newcrest, (Midtown Meadows in Bridgeview) which are the main houses i play with dogs. So my guess is it has something to do with the distance between stops, Chestnut Ridge is large and sprawling, while San Myshuno is compact, and the distances between stops are much smaller.
But the stops themselves I think are a large part of why walks can take a long time. I get the concept, they are walking to a spot (seems to be spawn points) and taking periodic breaks to allow the dog to do it's business. But those stops seem to be longer than they need to be, and if you pair it with a large map like Chestnut Ridge, walkies can take FOREVERRRRR.