On the Salkantay Trek, you will hike between 5 and 8 hours a day. The hike starts very early each day, so each hiker can go at his own pace.
There are some high mountain passes of up to 4,650 meters (Salkantay Pass) to overcome. The descents can be strenuous for the knee joints.
Especially the very thin altitude air brings many participants quickly out of breath. It is up to you to decide if you are up to the hike.
The travel agencies do not take any responsibility for this. You don't have to be a marathon runner or a professional hiker to do the Salkantay Trek, but a certain level of physical fitness is advisable.
Regardless, after arriving in Cusco, you should spend a few days getting used to the altitude and make sure you don't catch a cold or stomach problems from contaminated food.