When I stayed the night at the Matamoros fire station (just before Torréon) I had spoke with the fire chief who had biked to Mazatlan and took the same route that I later did. What was apparent from looking at the satellite map was that this was going to be a beautiful but demanding ride. Someone later told me that the Libre 40 highway from El Salto to Mazatlan is one of the most stunning roads in the world. Although I am not currently the most accomplished traveler in the world I think he could possibly be right