I run ultras and mountain races. I ran one this Friday night into Saturday morning - a 12 hour overnight trail race where I managed 100K (62mi) in 11 hours and 23 minutes.
Anyway, let's throw out willpower and even genetics for a second so I can bring up something: In today's age, virtually no one is even remotely close to their physiological limits.
Literally, hundreds of people (elite marathoners, a few ultra marathoners, cross country skiiers) out of billions of people are bumping up against physiology. These people will do anything for the smallest performance gains (1%).
Really, if you have not achieved a level where you are ~5-8% body fat and run consistently at 100 miles (or more) a week, you are not anywhere that close. Trust me. This is kind of even standard for competitive college distance runners - and even they are likely making mistakes that hold back their development (diet, drinking, running too many miles at too fast of a pace, etc.)
Yet we can take someone who is 20% body fat, and runs 10 miles a week. You will find this person finishing an 100 mile ultramarathon. It's not that uncommon, if the time cutoffs are liberal. 18 minute miles is walking pace.
The author even mentions he didn't think he was obese. He just didn't know the reality of things, I'd argue. He had to have this concept of willpower (concentration) to learn. And to change his diet and training habits. And even his understanding of what is possible.
It's magical to chalk things up to grit - but there is so much process of knowing how the lows feel during a race, what to eat while training, how fast to do each run while training, how much to run while training, how to stay hydrated while racing, how to get nutrition while racing, etc.
"Tough" or "in control of self" to me is too emotionally loaded.
Edit: I almost feel bad writing this because it takes some of the magic that gets marketed about it. But this is a science-oriented crowd and should be exciting to know there is more of an opportunity for the objective than a lot of people believe.