Our epic’s origin story takes place in a stadium that no longer exists, with the protagonist operating under a corporate structure that no longer exists and waiting to hear from a man no longer with us.

Yes, call it an epic.

For if it is remarkable when any general manager of a sports team lasts 20 years at one job, how else would you describe it when someone can clock two decades as GM of the New York Yankees with no end in sight?

On Saturday, that milestone will become official for Brian Cashman.

“To work 20 years in that city, in that organization, under that type of demanding ownership, I don’t know how many could do it in baseball,” Brian Sabean, the Giants’ executive vice president of baseball operations, said in a telephone interview. “You scratch your head that he’s got that kind of staying power, but he seems to have the personality for it.”

Cashman’s predecessor, Bob Watson, said bluntly, in a telephone interview, “I don’t know if he [Cashman] would’ve lasted 20 years if Mr. [George] Steinbrenner was still there.” Yet Cashman, who joined the Yankees full time in 1989 as a 22-year-old, did make it through a full decade with The Boss before Steinbrenner’s children officially took over. He has displayed remarkable survival and adaptability skills that have him not only still on the job after 20 years, but at the peak of both his powers within the organization and his standing in the greater baseball community. He recently signed a five-year extension to stay on board through 2022.

If not for media inquiries, Cashman insisted, “I probably wouldn’t even have been aware of it. I know this is my 21st season, but I wasn’t thinking about the [actual date]. I’m not nostalgic about it.”

While he lacks nostalgia, he has nevertheless retained the play-by-play of how his coronation went down. On the morning of Feb. 3, 1998, Cashman, in his seventh year as the Yankees’ assistant GM, was at the old Yankee Stadium participating in a conference call for the American League scheduling committee, back when the AL and National League functioned as separate entities.

As the call dragged on, Cashman noticed Watson repeatedly lurking around the entrance to his office. “It was clear that he wanted to talk to me,” Cashman recalled. Watson delivered the bombshell: After just two years on the job, winning the World Series in 1996 and qualifying for the postseason in 1997, he had notified George Steinbrenner of his resignation due to health concerns and recommended Cashman to replace him and become the 15th person in The Boss’s 25 years of ownership to hold either the GM title or the equivalent responsibilities.

“You’ve got a lot to think about, buddy,” Watson said to his assistant, according to Cashman.

For the next 30-40 minutes, Cashman attempted to “unring that bell,” he said, lobbying Watson to hang in there and keep the job. That didn’t work. And the reality was that, prior to those 30-40 minutes, Cashman had spent more than 10 years preparing himself for the gig, from his first Yankees internship in 1986 to joining full-time in 1989 after graduating from Catholic University.

“Nobody worked harder than him,” said Hal Steinbrenner, now the Yankees’ managing general partner, who got to know Cashman when he became a full-time Yankees employee in 1991. “He had great people skills. My dad saw a lot in him. If somebody would give it back to George, that’s the kind of person he really liked. Cash would give it back to him.”

“I can’t say that I remember him being so hard-charging,” said Sabean, who worked as a Yankees executive from 1984-92. “In some ways, you always worry about the younger generation getting ahead of themselves focusing on the next job, but he was very task-oriented. People took note of that.”

“He was my rules guy, the guy to make sure we had physicals and all those kind of things,” Watson said. “When I left, I told Mr. Steinbrenner that it would make sense for him to elevate Cash to the job.”

So George Steinbrenner called Cashman that morning, as Watson said would happen, and at the Regency Hotel on Manhattan’s Upper East Side, the Boss offered Cashman the opportunity of a lifetime. At age 30, Cashman became the second-youngest GM in the sport’s history at that juncture.

Now, at 50, four World Series rings and six pennants later, Cashman ranks as an elder statesman. Only Sabean, who became the Giants’ general manager in 1996, and Oakland’s Billy Beane, who assumed the same job for the A’s in 1997, have been with the same team longer as a baseball boss, and both men have assumed higher titles while hiring younger GMs to share the workload. In Yankees history, Cashman trails only Ed Barrow, who ran the team from 1920-1945.

It has taken until the last couple of years, however, for Cashman to get his due. Consider that Cashman had to last 18 years on the job, past Derek Jeter’s 2014 retirement, before he could say the entire roster had been built on his watch. Only last year, when the Yankees surprised the baseball world by lasting until Game 7 of the AL Championship Series, did Cashman receive full credit after remaking the organization’s farm system and executing a series of shrewd trades, thereby exhibiting that his accomplishments could be explained by more than just high payrolls.

“You put the body of work in any other place in baseball, or in sports, especially at the young age he started and the success he’s had, and he’s one of the top executives in the history of any sport,” said Sabean, who built three World Series-winning Giants teams from 2010 through 2014.

“We pride ourselves in aspiring to be a top shop,” Cashman said. “Sometimes you get more credit than others. There’s an ebb and flow to that. Right now, there’s more of a flow than an ebb.”

After saluting the Steinbrenners for their stewardship — The Boss died in 2010 after years of declining health — Cashman cited an organizational flexibility as the primary reason for the sustainability. The Yankees’ baseball operations department has expanded dramatically over the last two decades as it adjusted to the analytics age and scientific advancements in nutrition, sleep and weight training, among other areas.

“I think like in any industry, if you want to be successful, you have to be open-minded to evolve and change and grow and adjust,” he said. “We do everything possible to make sure we don’t die off with the dinosaurs. You keep your eyes on success stories in all areas, whether it’s Netflix, the San Antonio Spurs, what makes [Kentucky coach] John Calipari tick. You watch the success of the New England Patriots and see if there’s any secret sauce.”

Asked to recall his superlatives and lows on both the team and individual levels, Cashman went dark on the first matter and light on the second.

“I look at lost opportunities,” he said. “ ’04, we let that get away [in the ALCS collapse to the Red Sox]. The 2001 [World Series] situation, Game 7 with the lead in the ninth inning and we lost, that’s almost impossible with the greatest closer ever [Mariano Rivera]. … The first-series knockouts [2002, 2005 through 2007 and 2010 and 2011]. I look at the missed opportunities and the hardware we could have delivered more than when we finally did win.”

He expressed pride in the trades for Roger Clemens, David Justice and Alex Rodriguez and the signings of international stars Orlando Hernandez and Hideki Matsui. On the flip side, “It’s hard to point to any one [bad move] specifically,” he said. “Certainly I’ve got my blemishes that, if I could do a do-over, I would. You take the good with the bad and deal with it, look at any areas that were deficient and try to evaluate the mistake.”

For instance, the disastrous signing of Kei Igawa to a five-year, $20 million contract on top of a $26 million posting fee to the Hanshin Tigers prompted the Yankees to overhaul their Japanese scouting system.

“I don’t think we knew enough at the time about the ball differences, or pitching once a week in Japan versus every five days here,” Cashman said. “There are different factors and an education on those that has improved our process.”

An improved process and the Steinbrenners’ continuing financial commitments have Cashman and the Yankees primed for what they hope is another run of title contention with a mix of homegrown studs like reigning AL Rookie of the Year Aaron Judge and outsiders like reigning NL Most Valuable Player Giancarlo Stanton, who arrived from the Marlins in a stunning December trade. If the Yankees win another World Series or a few, the run will be a complete Cashman production, with minimal interference from ownership. Days before acquiring Stanton, he introduced Aaron Boone as the Yankees’ new manager, a high-risk, high-reward maneuver.

It’s not the praise that drives him, Cashman insisted, but rather how sweet it feels being on top after working collectively for it.

“Ultimately, what drives me is needing another ring,” Cashman said. “I have rings. I’ve got five [including 1996 as assistant GM]. It’s pretty simple: I need another ring. Create a hashtag for it: #NeedAnotherRing. That’s what I want.

“I keep it simple. We’re not putting on a concert 81 dates a year and entertaining the fans. Our sport is about wins and losses.”

That, surely, is a philosophy that would please The Boss. Asked how his formerly trigger-happy dad would feel about one GM lasting 20 years, Hal Steinbrenner said, “I think he’d probably get a chuckle out of it.”

Most epics feature a few laughs. Drama rules these stories, though, and if Cashman’s efforts produce the sort of drama he desires, the conclusion to this saga might take place at a locale where no concerns exist about shelf life: The Hall of Fame.