It seems so out of character for him looking back now after all these years and after all these yards:

The day Frank Gore thought about quitting football.

Running backs aren’t supposed to endure 15 years in a collision sport that will, more often than not, break their bones and break their will, but then Frank Gore listened to the doubters and naysayers who counted him out, only when he could use their disrespect as fuel to light the fire inside him once he kicked the NFL door down.

And 15,021 yards later, Frank Gore continues his inexorable march to Canton, on 36-year-old legs that refuse to stop churning.

He gets lost in the shuffle when we talk about wondrous Saquon Barkley and his recuperative powers, and dual threats Christian McCaffrey, Alvin Kamara and Le’Veon Bell, and relentless Ezekiel Elliott, but it is so wrong.

Because when the Marines were looking for a few good men, they should have started with Frank Gore.

“First I gotta say it’s the man up above,” Gore told Serby Says by phone. “I’ve been blessed. And also the love of the game, and I work hard, man. I try to work out with a lot of young guys in the offseason to see where I’m at.

“You gotta love it, man. You gotta love it.”

Except there was that one time when Gore suddenly stopped loving it.

Former University of Miami running backs coach Don Soldinger recalls a fateful practice when Gore blew out his knee and wound up missing his sophomore season.

“[Future Redskins safety] Sean Taylor just jumped in the hole, and didn’t even hit him, and Frank made a quick cut,” Soldinger said by phone.

In a Thursday night game against West Virginia the next season, Gore suffered his second ACL tear.

He had overcome a learning disability at Coral Gables High School. He had shared a nine-bedroom home with nine other family members. His mother, who had been addicted to cocaine, was on dialysis.

At Doctors Hospital on the Miami campus, Gore surrendered.

“I thought I was gonna stop playing,” Gore said.

Soldinger was with Gore at that moment.

“He said, ‘Hey, I ain’t gonna play anymore.’ This is ridiculous,’cause he had just rehabbed his other leg, you know?” Soldinger said. “He got real down in the dumps and said, ‘That’s it, I don’t want to play anymore.’

“I said, ‘What? C’mon man, you gotta be kidding me. You’re so talented it’s unbelievable.’

“Dr. Uribe, who was our physician, had said his ligaments were too small for the grooves in his leg, and he says, ‘That was gonna happen regardless. His legs are gonna be better now than they ever were.’ ”

And they were. With the 49ers for 10 years, then with the Colts for three years, then with the Dolphins last year, even now with the Bills.

It is at the otherworldly Bommarito Performance Systems in Davie, Fla., where, from the time Gore left the U. of M., his maniacal obsession with stiff-arming Father Time is legendary.

“It’s a great thing to watch him not just go through the training for himself, but mentor these younger players coming in about the difference between training hard and training smart,” Pete Bommarito said by phone.

“One thing that’s kind of unique is he likes to box. We introduced him to that, I think, maybe around 2008, 2009. We have a boxing program in-house, we train a ton of pro fighters, and everybody that’s ever watched him hit the bags and do hands work and spar and things like that said if he decided to quit football and just go right into boxing, he wouldn’t miss a beat. He’s that good. He’s a beast in the ring.”

He’s a lean, mean, conditioning machine whose regimen is not football-specific.

“Frank’s just different. He buys into what we believe in, and that’s train for your body,” Bommarito said.

Gore has 61 carries for 273 yards and two touchdowns in 2019.

“I try my best to do everything right,” Gore said. “I try to impact the game every type of way I can.”

Giants safety Antoine Bethea entered the league one year after Gore in 2006.

“Hard-nosed runner,” Bethea said. “One of the hardest runners I’ve played against. And one of the things with Frank is that he’s always falling forward.”

Bethea was Gore’s teammate in San Francisco in 2014.

“Not gonna say a lot,” Bethea said. “He shows you how to be a pro, in the weight room, practicing every day, not taking any days off, take care of his body. You can count on him every Sunday, man. Consummate pro, a leader and one of the greatest backs to ever do it.”

Bommarito still cheers his friend on from afar.

“I love every year how people try to finally say, ‘OK, this is the end of the road for Frank, let’s draft him low in the fantasy draft,’ ” Bommarito said. “What does he do? He goes out on Sunday and just rips it.”

I asked Bommarito if he has ever met anyone as driven as Gore.

“I mean, obviously, I have tons of professional athlete clients that are all very, very driven,” he said. “I just think what makes Frank special is he’s just got that nonstop chip on his shoulder.

“From the day I met him, he’s pissed off because people are questioning should he come out early. And he’s had the knee surgeries in college, and can he even play in the league? It’s nonstop bulls–t, pardon my language. But he just loves to constantly prove people wrong. It’s awesome to watch.”

Indeed, when you ask Gore what he is most proud of, he says: “I would say I’m still getting an opportunity to prove people wrong. I’m still having success and helping my team be successful.”

How much longer does he wants to play?

“I really don’t think about it,” Gore said. “I look at it week-to-week every Sunday.”

Gore trails just Emmitt Smith (18,355 yards), Walter Payton (16,726) and Barry Sanders (15,269). I ask him about the Hall of Fame. “Hopefully I’ll get in,” Gore said.

I tell him he deserves to get in.

“Appreciate it,” Frank Gore says.