Nationals hitting coach Kevin Long, the former Yankees and Mets hitting coach, is going to his third World Series since 2009, in large part because of the work he did over the winter with Nationals hitters, most notably veteran Howie Kendrick.

That 2009 season was the last time the Yankees won a World Series, and they have not been back since.

Long’s visits to his players in the offseason often pay huge dividends. That is one of the many reasons he was so successful with the Yankees and with the Mets in 2015, the last time they went to the World Series.

Long, 52, nearly got the Mets managerial job that went to Mickey Callaway, making this trip with the wild card Nationals to the World Series, while the Mets finished out of playoff contention and are again looking for another manager, even sweeter.

Long and assistant hitting coach Joe Dillon made pivotal adjustments with the Nats during the season, getting the hitters to focus more on putting the ball in play, and in the Nationals’ sweep of the Cardinals, they scored seven runs in the first inning of the 7-4 clincher without the benefit of a home run, a rally that included three opposite-field hits, a sacrifice fly and a sac bunt.

Contact counts for the Baby Shark-lovin’ Nats.

Kendrick, who was named NLCS MVP after hitting .333 over the four games, is quick to credit Long with the success he had this season. It all started with those winter workouts.

“I’m just trying to get smarter,” Kendrick, 35, said. “Making adjustments, I would say, is the biggest thing. Trying to be more efficient with my body and my swing. Kevin Long is a big part of that. Kevin lives out in Phoenix, and I live in Phoenix. It’s funny because the first time we met, I said to him, ‘Hey, what can I do to get better?’ He had a list, like he had wrote down on a pad of paper. I wasn’t expecting it. This was the first time I had ever hit with him. He had this sheet of paper. He goes, ‘Alright, this is what I know about you. This is what you hit with this, this, and this.’

“We got to work from there. Last year was bittersweet because I got off to a good start and ruptured my Achilles,” Kendrick said. “Having the ability to come back this year and be a part of this team and to be with the guys in the locker room, that was huge. Me and Kevin and Joe Dillon, we got to continue the process that we’d already started with my hitting, and I just trusted them and stuck with it. They just helped me get better at a time when I really needed to.”

Quite the endorsement from a highly respected veteran. Other veteran hitters such as outfielder Adam Eaton sing Long’s praises. Long, meanwhile, is quick to share credit and said Dillon is “the best assistant in baseball.”

“These guys mean so much to me,” Long told The Post of the Nats, pointing first to the overall greatness of superstar Anthony Rendon, one of the most complete hitters in the game. “They are so much a part of my family, and the time and effort they have put in from Day 1 and to see a lot of the fruits for a lot of the layout, it’s a really good feeling, especially to know where these guys came from. We were sitting at 19-31, and I remember us having meetings talking about, ‘We got one of two things we can do. We can fold up the tent right now or we can battle our asses off.’’’

They battled and now will have to face the mighty Astros and Gerrit Cole in Game 1 Tuesday night at Minute Maid Park.

The further you travel in the postseason, the rougher the road.

“It’s pretty special,” Long said. “It’s really a group effort, it’s not just one guy. It’s Michael Taylor going down to Double-A, working on his swing and getting that right. It’s Adam Eaton coming off serious injuries and not having his legs last year to get right. It’s Trea Turner working his tail off.”

Kendrick’s 10th inning grand slam to shock the Dodgers in Game 5 of the NLDS was the biggest hit of the postseason for the Nats as Kendrick somehow got to a 97-mph two-seamer down and in.

Noted Long of the changes, “I got a text from Jeff Pentland [who replaced Long with the Yankees and was once the Dodgers hitting coach], and he said Howie Kendrick couldn’t hit that pitch, the home run he hit to center. He said it was an inside fastball that he couldn’t get to before.”

Kendrick crushed the pitch to center and now, after sweeping the Cardinals, the Nats own six straight postseason victories and are looking to shock Houston.

“Howie worked really hard on his direction and made sure he wasn’t striding inward, and that freed him up in there,” Long said of the key adjustment Kendrick made.

“They just never quit,” Long told me of his hitters, noting the seven-run, ninth-inning rally they produced to beat the Mets in early September. “Every at-bat is important to them, and when every pitch is important to them, things like the comeback against the Mets can happen. They fight for every inch.

“I’m in a good spot,” Long said of no longer being in New York with the Mets or Yankees. “My main goal is to help hitters become consistent. Yeah, it’s a different uniform in a different city, and I have fond, fond memories of my time in New York.”

He smiled and added, “The memories are starting to build here, too.”

Perhaps, like in 2009 with the Yankees, Long’s team will win another World Series.