We took the train from TDI and the 6:00PM or so train back to Ollantaytambo. I didn't have tickets to Machu Picchu, so by the time we got to Aguas Calientes, I found an ATM, got the tickets, got bus tickets, and got up there, it was well after 10. I hurried to the Montana Machu Picchu and still got there just after 11. When I got back down, we had lunch then spent about an hour and a half with a guide. We left just before MP closed and had a few minutes in Aguas Calientes before we had to get the train. It was no problem getting a taxi back from Ollantaytambo in the evening. Know the rate, though, since the opening offer from the cabbie will be crazy high.
Anyway, as others have suggested, this is going to be one of the highlights of your trip. I think you'll want some extra time and you won't want to be watching the clock the whole time you are there.