How the day goes
According to the tour listing, the day lasts 13 hours from hotel pickup in Cusco to the return. It includes bus transport, a guide, breakfast and a buffet lunch. Expect a long day with an early start, so do not combine it with another excursion on the same day.
What is included and what you pay separately
The listing includes hiking equipment, blankets, emergency oxygen and a first aid kit. Entry to Mollepata village is not included and is paid separately, according to the listing. Bring cash for that payment and for your personal expenses, and ask the seller when during the day it is collected.
How much effort it takes
There is a hike to the lake, which is why the tour includes hiking equipment. Wear closed shoes with good grip, layers of clothing, sunscreen and water. If you have doubts about your health or fitness, see a doctor before you travel and ask what they recommend for your case.
When to do it on your trip
Book Humantay after your first few days in Cusco, not on the day you arrive. Sleeping a couple of nights first gives you time to acclimatize, and the altitude sickness page explains how to plan it. Keep it on a different date from Machu Picchu, because each departure has its own logistics.