I Can't access the dessert route to get the slither route as I am block by something and if I go down, I would return to my previous location
I don't know if this is still an issue but it took me a while to figure out too! You just have to carefully maneuver the horse up north on that stretch of dirt path, then left/west, and down and you can get to the lake. It takes a bit of extra clicking to get there because it's a narrow path I think!