There are a lot of questions for this method, but does not fit in my case. I have designed a network of roads, manually, and this is optimal. Max longitudinal grade of roads is 12%. (These are forest roads). I tried to calculate least cost path with r.walk, SAGA Least cost path, and I got a result like this.
On this link I have set an example of the whole project with layers [http://www.mediafire.com/download/me8vgx77vu7vtzw/LeastCost.rar], so if someone wants to try. If someone has a good example to explain it would be good.