Go on a solo twelve hour walk with no distractions, and you'll learn a lot about yourself. You can do it, even if it's hard.
The 12-Hour Walk is two things: a self-help book, and Colin O'Brady's memoir. He's an interesting person, but as far as the self-help element goes, the above summary will either resonate or not, but that's basically the book. Overall, the book felt like a mess to me. I'm sure it was exactly the kick many people needed, so I can't entirely dismiss it.
But the book didn't know whether it wanted to be a memoir or self-help book, and the self-help element felt exceedingly thin. It is no sermon from David Goggins, and the memoir itself was also just fine? Overall, I recommend the practice (it's indeed useful!), and do recommend videos about him, but think the book can be skipped.