The walking trail sounds like fun! There is not one right answer to your question to fit all dogs.
When my boys start losing focus and try eating things more than walking, then I know they have had enough.
Some Yorkies will just slow down or sit down when they are tired.
Max started with 1 mile at a time comfortably when he was a pup. Sometimes we would do 2. He usually sacks out for a bit when we return home. Teddy always has happy feet and walks around the house for a bit before settling down for a rest.