I follow its principles if not the actual detailed program. He says to move often at a slow pace, lift heavy things, and sprint now and then. So I take walks or hike most days, lift weights 2 or 3 times a week, and sprint 1 or 2 times a week. I have found it's enough to keep me in shape for super strenuous backpack trips. 20 miles in one day is not a problem.
Opposite most people, I thought lifting weights would be easier to start with than doing bodyweight exercises. I really couldn't do most of the basic bodyweight exercises. I started with a class that had us doing them but I felt so uncoordinated and even after almost a year of the class I felt I had made little progress. So I switched to barbells. It seemed simpler (load barbell, do lift, load more plates, do more lifts = less complicated than turn your body into a pretzel and hold it in mid air somehow) and I saw dramatic progress quickly so it was kind of addictive. From lifting weights I can do 10 or more real pushups in multiple sets (with rest in between) without having tried to program pushups.
I may switch all my upper body work from weights to pushups, pullups and dips. Unfortunately there is nothing to replace squats or deadlifts with barbells. I will keep squatting. I don't know about deadlifting. Too dangerous for uncoordinated idiots like me.
Female, 5'3", 50, Starting weight: 163lbs. Current weight: 135 (more or less).
I can squat 187.5lbs, press 75lbs and deadlift 200lbs