As Peyton Manning exits the stage, the hard part for him likely begins. Manning has been among the best at his profession since he was a teen, and now he begins a new chapter. (Patrick Smith/Getty Images)

What Peyton Manning does next will be the hardest thing he has ever done. With the announcement that he is ending his NFL career just shy of 40 years old, Manning will surrender the abject hero worship and the trophies he believed were his perennial due for the status of a bystander. How to slip into an ordinary life? How to become a slump-shouldered older man with a thickening middle instead of a loose-limbed perpetual boy?

He has never been anything but great, ever since he was a middle schooler growing up in New Orleans as a second-generation star quarterback, never ended a season without an ovation. He played 18 years in the league, long enough that he started watching film on Betamax and finished watching on an iPad. Long enough that the arm became lank and lax. “It’s like stepping on the gas and there is no gas in the car. It’s simply a waste of time,” Manning told me a couple of years ago, and that was when he still had a little something.

In the Super Bowl a month ago, it was visible he had nothing left but the know-how in his head, on top of that surgically repaired neck. You could see the difference between Manning’s throws and the forceful larger-caliber ones of 26-year-old Carolina Panthers quarterback Cam Newton. Manning’s took so long to reach a receiver you could practically count “One Mississippi, two Mississippi” from the time he let the ball go to its dying arrival.

Still, there was a dignity in how he wore out that arm. Manning played until he was past it, and the reward was not just two Super Bowl appearances in his last three seasons with the Denver Broncos but proof that his inherited genetic gifts and extravagant statistics were the least of him. His critics could no longer say he had more numbers than real accomplishments. The stats are just gilding: He goes out as the NFL’s all-time leader in passing touchdowns (539) and yards (71,940), but more important, he’s the only man to quarterback two franchises to Super Bowl victories, and he took his squads to the playoffs in 15 seasons.

In the last three seasons, Manning totally redefined himself, from a golden arm to the greatest mentalist to play the game. As that weirdly slack throwing motion weakened, it became obvious just how much his cerebral, scanner-like play-calling and obsessive-compulsive preparation mattered. NFL Network analyst and former Baltimore Ravens scout Daniel Jeremiah tweeted a story about seeing a defensive line coach in the office at 1 a.m. and asking him when he was going home. “We play 18 this week, no time to sleep,” the coach replied. Bruce Arians, Manning’s first quarterback coach, recalled meeting Manning for a pre-draft interview in 1998. Manning showed up with a notebook full of questions written in neat script, “including one about the Indiana tax code,” Arians said.

This season was a fascinating third act in which he delivered the worst physical performance of his career. He was hurt and humbled, and he had the mettle to keep working, to ignore public humiliation and other people’s opinions. When he was benched after throwing four interceptions on a torn plantar fascia in his foot, it wasn’t a corrective like it is for young players. It was a glimpse of the inevitable end. But he managed to win the job back from Brock Osweiler and did enough with his mind to help the Broncos in the playoffs and then rode the coattails of the greatest defense of the epoch to another Super Bowl trophy.

Still, it will be the fourth act that’s the hardest. We’re all great at the thing we love to do most. From now on Manning will have to perform at the things he is not so good at, that require the quiet tying of a necktie and the pulling on of a pair of loafers to face another mundane day, maybe as a TV analyst with a microphone, maybe as a coach, but either way yielding the role of prime actor.

There will be a sort of de-escalation. It already has begun, with the furor over a decades-old and perhaps exaggerated allegation that he sexually harassed a trainer at Tennessee and the more recent one that HGH shipments were delivered to his home in his wife’s name while he was recovering from neck surgery. Does it really matter that he showed his butt as a college boy or what he used to heal his neck after fusion surgery left him with a zipper scar and so much nerve damage he couldn’t make a dart stick? It will matter less and less the further he gets from the field.

The on-field Manning was a situational personality, an immortal one. Off the field we really don’t know who he is, except to say he has been always courteous, available and an intriguingly funny, deft actor in commercials. But now comes the more revealing part, the fashioning of a new self in the face of the boredom and mental drift of retirement, the loss of a reassuring regimen and purpose that make it so difficult for great athletes to walk away. What else, after all, are they going to do with their days? “The harder you work, the harder it is to surrender,” Vince Lombardi said.

Sometimes, it turns out the greats are not such likeable people, forever dwelling in a state of don’t-you-know-who-I-am narcissism. But sometimes they turn out to be genuinely nice people and good company, though not at all in the way you expected. Retirement sometimes leads to candor and self-deprecation. A Deion Sanders is more truthful and self-aware, a Joe Gibbs is more witty, a Steve Young more observant and loquacious than you would ever suspect. It will be interesting to meet a more mortal Peyton Manning and to see who he turns out to be.

For more by Sally Jenkins, visit washingtonpost.com/jenkins.