I'll ask my friends in Cuzco and see if anyone has ridden it. But be forewarned; the Urubamba is a very large steep valley. I don't know if you can get a sense of the scale on google earth. Even if you can ride that route, you'll be hurtin' fer certain. But you'll always have the option of turning back to Urubamba and cranking up the asphalt to Chincheros which would be challenging enough for most riders. I'll ask around and get back with you later.