I regularly participate in events known as GORUCK Challenges. They're 12 hour+ endurance events led by current and former special forces operatives from all branches of the military.
Yes, there are plenty of young extremely healthy men at these events, but they're the minority. The majority of participants are over 30 years old, in good but not great or extreme condition, and tend to be 10-30% female.
These events are hard. Hours of PT, carrying of heavy objects for hours at a time, all while wearing a backpack that weighs at minimum 25#.
Success rate is about 94%. Yes, there is some principle of self-selection at work whereupon if you're dumb enough to sign up for such a thing you're predisposed to survival, but the biggest reason that participants finish is because we are all capable of so much more than we think.
That's the point of the article, IMO. Don't try harder, just don't fucking quit. Hell, try less if that's what it takes to not quit.
Edit: accidentally hit enter too soon. I wanted to say something about "not training hard enough" not being the issue with most athletes I see, rather they're training too hard with poorly designed programming.