The big takeaway I got from this book is that it talks about how to maintain your conditioning over a lifetime. (Dan John reserves the word "fitness" for how well a person's strengths match the sport or athletic endeavor they are working toward.) I would say that the theme of the book is the fitness industry's products are a bad fit for most people for most of their lives.
Some specific takeaways are:
Most of the time, your workouts shouldn't take a huge part of your day.
Most of the time, food routines are going to be a better investment in maintaining your weight than strict diets.
Most of your strength-building time should be spent on hinging, pulling, and loaded carries and not the mirror-friendly pushing exercises like curls and bench press.
John uses the metaphor "park bench vs bus bench workouts" and also "warrior mentality and king mentality". He says that for most people who aren't young professional athletes, most of our time is going to be on the park bench, or working from the king mentality. This is the baseline our habits set and reinforce. We have the capacity to have up to two bus bench/warrior sprints a year but those should be exceptional. For me, this was the big take away. As a fitness consumer I'm bombarded by weight loss challenges; weight loss solutions; boot camp challenges; HIIT clubs; Insanity; Viking Fitness programs; Theories; and so many more. It is a relief to hear that sustainable habits are enough.
John originally presented this information at a seminar for trainers. Having that context helps explain some of the downsides I found in the material. Sometimes he is referring to another set of information and neither the book nor the audiobook are always clear about what that information is. In other places he's clearly directing the reader to another book or program. Some specific things I recall are the his assessment protocol, his "ready to go" book, the Simple Strength book (with Pavel), and the Velocity diet.