The trek was a great experience and is a must if visiting Peru. The staff that we had for our 5 days were really nice and full of knowledge throughout the whole trip.
The first two days were the most difficult, especially the morning of the second day when trekking to over 4600m above sea level. The next 3 days were a little easier at lower altitudes. The weather also improves the closer you get to Machu Picchu, we were lucky with the weather but you need to be prepared for unexpected changes in the forecast.
There is no need to book online in advance as it is way more expensive than just signing up for it when you arrive in Cusco.
Overall a fantastic experience and would recommend it to everyone.