New England Patriots quarterback Tom Brady winds up to pass during an NFL football training camp practice, Thursday, Aug. 1, 2019, in Foxborough, Mass. AP Photo/Steven Senne

At this point it’s fair to say that Tom Brady’s personal science project is going well. He just celebrated his 42nd birthday with a new two-year contract extension, and he apparently runs faster than he did at 22, his legs limber and alive, which is to say nothing of that ring-heavy throwing arm. In his 20th NFL training camp, he’s so radiant with health that he looks like he’s got spring water for blood and years left.

Clearly, scoffers who called Brady quackish owe him an apology. He was right, and they were wrong. He was way ahead of his time in perceiving the evils of an NFL culture based on heavy weightlifting and painkillers and ditching them for a range of unconventional training methods. His elastic, tenacious body is the empirical proof, more convincing than any lab result. Of all of Brady’s time-defiant feats, maybe the most impressive one is his physical resilience over the past decade from the age of 32 onward: In nine of his past 10 seasons he has started all 16 games. The only thing that could keep him off a field was NFL Commissioner Roger Goodell’s tyrannical suspension in 2016. So laugh at Brady’s avocado-chia smoothies if you want, but his habits line up with everything scientists know about anti-aging and avoiding illnesses such as arthritis.

None of it, however, explains how Brady keeps his mind right. How does he fight mental burnout and keep summoning the readiness to go through another August training camp? Years of success do not breed freshness in a quarterback. They breed expertise, regulation, know how. The wild raw physical music of a Patrick Mahomes is gone and so is the humbleness that begs to learn. What’s left is pure tedious process. Mental exhaustion is what really gets most veteran quarterbacks, not the physical inability to perform. They get sick of it and choose liberation. What keeps Brady from choosing to surf in Costa Rica?

Compounding the challenge is a phosphorous fame that makes everything a little more draining. Sure,he’s a walking jewelry store with his Tag Heuers and six Super Bowl rings, but it comes at a price: He’s such public property that even a routine dog walk turns into an energy-sapping Sharpie party. After his first Super Bowl victory, Brady accepted every invite and endorsement deal. After the second one, he went to Europe – by himself.

His career has become a demonstration of psychological pacing as much as physical. He’s all about conservation, the marshalling and portioning of resources and efforts, and not wasting energy on stupid things. In camp the other day, a crush of reporters wanted to know why he put on a few pounds, and where his latest contract extension stands.

“That’s up for talk-show debate,” he said. “What do you guys think? Should we take a poll? . . .

“I’m just trying to take it day-by-day and get out to practice. That’s what football players do.”

There’s no recipe book or training method that can illustrate his ultimate feat of endurance, which is to sustain the striver’s mentality with which he played as a younger man. You get the feeling that Brady’s willingness to work like another drab just trying to make the team is the beating heart of the Patriots franchise. It’s why they’re able to play each season as hungrily as if they’ve never won anything.

A long time ago, he could have turned the Patriots into servants carrying an aged king. We’ve seen how a fading Michael Jordan and LeBron James unequally weighted their franchises. So far, Brady seems to have avoided that. “He’s no different than the men left and to the right of him,” cornerback Jason McCourty observes.

Brady long has said that his main driver is “insecurity.” Back in 2005 he remarked, “I guess I always feel there’s someone hunting me down, someone always right on my footsteps.” It would appear that in a strange way aging only has enhanced that competitive anxiety, given him one more thing to outrun, a little more motive rather than less.

Yet that’s not the entire explanation either. Something more than insecurity, or science, or method must keep Brady marching on in this league, because it’s just too hard. On some unseen level, he must be aging, and feeling it. He’s not immune from the elemental biological processes that conspire to wither all of us. As Frederick Buechner has written, aging is “like living in a house that’s in increasing need of repairs . . . Cracked and dusty, the windows are hard to see through, and there’s a lot of creaking and groaning in bad weather.”

And that’s without absorbing the battering of the NFL. Beneath Brady’s elegant attenuated figure, there must be groaning, and an unimaginable effort going on. What explains that?

Maybe the secret to Brady is simple: He’s just a man who loves his work.

“You know, it’s hard for me to imagine doing anything else in life,” he said the other day.

Strip away all of the gloss, and the greens and the gluten-free, and what’s left is someone who can’t bear to give this up. Under all the cool mechanistic excellence, that’s the core of Brady’s palpable ferocity, his insistent curiosity about training methods, his determination to find the last limit, and thoroughly exhausted himself. It’s fascinating to watch – and instructive. It’s an all-inness worth envying, that makes you say, “There it is. That’s what real love of the game looks like.”