Okay so we are agreeing that Tiger Woods is a massive product of his environment right - I think anyone that watched the HBO documentary would agree on that. And sure - I used the most extreme example in terms of an athlete there has ever been.
We can totally agree to disagree on the part of especially teenage races being decided by mental toughness. Because by these ages the physical talent is rising to the top it is the ones that are harnessed the best in the face of a million different other things in life going on (school, sport choices, drinking, romances etc etc etc) that are your winners and this is not a physical thing. For me, being able to navigate all these things and be dedicated enough to physically evolve oneself to the level you are the best on race day (and hence "decide" the race) is a tangible that isn't physical.
Here is an interesting article...
Kerr’s innate steel was forged in Edinburgh by his father, a former professional rugby player, and mother Jill, a physio. “I just wanted to be the best in the world at something,” Kerr says. He wanted it to be rugby — his brother Jake has played for Scotland — but by the age of eight he was shedding other sports.
Couple of key takeaways - he dreamt/had aspirations of being the best in the world at something - which even though the age isn't given when these began it's not unusual for these dreams - no matter how lofty at the time, to start forming even as a pre-teen. And by the age of 8 was beginning to already "shed" other sports (presumably to focus more on what was likely his best sport at the time - track).
Fitness, which determines success in this sport comes from the ability to harness and maximize ones talent and the snowball effect of years doing so. The resiliency to keep at this is far beyond a set of lungs.