The Salkantay trek was great. It was though but when we made the top it was worth it. Definitely recommend this trek.
The guides knew our names immediately and they told our fun stories en useful information. They were also very careful when some of us get sick because of the altitude. The food was great and also the facilities were ok as well. We did the 4 days trek and I think it's enough. After day 3 every muscle in my body hearts.
So you have enough energy to go to Machu Picchu. We also went to the hot springs, they were that kind of flexible to arrange that for us!