Okay, so here's the confusion. If you look closely at that image, running from bottom to top thru the center, there is a ravine. At the top of the ravine, you can see the white blob of the salt pans and if you look closely to the right, you can see a vague white line parrallelling the ravine. That line is the single track. It is a lot of fun on and unloaded bike, steep, technical, but fun and it continues on down to the valley floor. That single track becomes a gravel road further down as you approach the Urubamba river and you ride thru a lovely eucalyptus grove, cross the Urubamba and pick up the pavement. I was confused b/c I thought you were attempting to ride that single track in reverse. That would be brutal, you would be walking most of it up to Moray/Moras-very steep climb. So if you decide to spend the night in Chincheros, you could explore the area, lots of nice STs and you could do a smaller loop within your bigger loop. That smaller loop would be Chincheros, pick up the ST to the salt pans and down back towards Urubamba were you would pick up the pavement back to Chincheros. Of course, that would mean cranking up the pavement twice from Urubamba to Chincheros and for most people, once is enough. You had me worried, I thought they might have paved that single track! Unlikely since that whole area is an INC archeological zone and they restrict development in those areas. The only road changes that I can see from your image since I've done that route is that they paved the small bit from the main road to Maras-that used to be gravel-and now it's paved to Maras. If you don't want to do the smaller loop within your bigger loop, when you ride up to Chincheros just take the turnoff to Maras and explore the area. It's really interesting to see how the Incas made salt and there is a cool circular terrace that they made to experiment with different crops. Also very nice scenery in general and nice riding.
I lived there from '85 til '97 but have only visited since. I'm doing the Salar this fall and since I have the frequent flyer miles for a free trip to Lima, I'll probably go to Bolivia via Cuzco. My carrier does not fly to LaPaz and besides, I really need to see some old friends in Cuzco. Hit me up w/ more q's if you want.

PS There is a thorn bush in Peru that seems to shed it's thorns. You will find these thorns in your tire so when you fix your flats, always make sure you examine the tire in the area of the wound and pull the thorn out. Otherwise, you'll fix your flat, get on and ride 50 yards and have another flat as the thorn re-punctures your tube. Better yet, get a tire with puncture protection. You'll probably still get the thorns thru your sidewall but they will definately help.