A tough and very rewarding hike for the over 50 set. The second day is touted in advance as very difficult, and though that's somewhat true, it is certainly well worth the effort.
The glacier is gorgeous and you get beautiful views of it all morning. The hike to the lagoon on the first day is a great warm-up and a fantastic destination in its own right.
The morning hike on the third day took us through varied micro-climates, each interesting. And, of course, the climax of the trip is the visit to Machu Picchu. Best to get there when the doors open so you can enjoy first light and relatively fewer tourists.
Highly recommended! Go before the Salkantay trail gets too commercialized.