In the stretch between Hidroeléctrica and Aguas Calientes, the path is essentially the railway line. It might sound unimpressive, but it's actually a great section to walk around. The train rarely runs and the route is very remote, with few places to stop, so you're basically walking right through the middle of the jungle. The sounds of wild animals are amazing, especially birds. You can also see beautiful wildlife along the way if you look closely in the forest.
The walk itself is very easy, completely flat. Another important point is that if you look carefully, you can see Machu Picchu almost all the way.