Oooo, I really like the separation of mentally and physically difficult above, and the mud is something we often forget as well.
My first walk into Burgos was a nightmare all along the busy street on sidewalk. I was by myself. The second time was barely noticed because I walked with two others at a good pace and full of laughter. Both late morning, summer.
If possible, consider leaving a couple of days "open". I know there is a better term but can't think of at the moment. Sometimes our Camino family will play a role in our journey whether walking a bit faster, a bit slower, or not at all. It's nice to have this option as well, but I understand sometimes it's a matter of catching a flight home on a certain day.
Also keep in mind how one's body usually becomes stronger as each week of walking goes by. What might have destroyed the muscles in the first week, may not even be a concern in the fourth.
A valuable lesson for me was to listen to my body not my ego.
May you have the best of all journeys, TammyLynn.