For Brooklynite Lauren Rutkowski, 34, the day of the TCS New York City Marathon is perhaps the most important on the calendar — one she starts early in the morning, carb-loading on bagels near her home in Cobble Hill before hopping the subway to where the action is.

But the marketing manager won’t be running the five-borough race.

Instead, she’s one of 2 million New Yorkers and out-of-town spectators expected to show up bright and early this Sunday to cheer on a record-breaking 55,000 runners from all over the world, shouting encouragement from behind police barricades.

It’s an experience, Rutkowski, whose favorite viewing spot is near Central Park, said she wouldn’t miss for the world — one of the few times each year when New Yorkers put their differences aside and come together for a common goal.

“There’s something so wonderful and community-based about marathon day. Everyone is happy for everyone else, and you see all of the best parts of New York,” the superfan told The Post.

Many say marathon day is one of the best days in New York. Paul Martinka

And runners aren’t the only ones training all year for the big day — there’s an art to watching, too. That includes day-of rituals followed by people like Rachel Naurath, 33, who wakes up at 6:30 a.m. on marathon day morning to pop across the shuttered intersection of Fourth Avenue and Baltic Street to grab a coffee — and “post up with enough time to catch the first waves,” she told The Post.

“The marathon makes me so proud to be a New Yorker,” Naurath said. “It reminds me what a gift it is to be a part of this crazy community.”

The Boerum Hill resident first got hooked on the spectator life back in 2020, when she was cooking breakfast in her kitchen and “feeling sorry” for herself, she confessed. She was about to grumpily yell out her window at people shouting on her street — until she realized what day it was. 

Every year, Naurath looks forward to making fun, witty signs to cheer on the thousands of runners. Rachl Naurath

Now, Naurath hosts a group of people to watch the annual event near her home — a “great spot that’s never too crowded” — and brings blank posters, Sharpies and bagels.

“It’s my job as a spectator to be happy and engaged (and possibly a little drunk),” she joked.

There’s also a science to watching the marathon, superfans say — partly due to the difficulty of navigating street closures along the route, but also to knowing the best spots to catch the runners, who will experience a vast range of emotions along the 26.2-mile course.

Bagels and mimosas on marathon morning are quintessential New York. Rachl Naurath

Big fan Bobby Westside, 34, told The Post he likes to set up shop at around mile 16/17, after the runners cross back over the Queensboro Bridge into Manhattan.

“It’s that critical make-or-break moment in the race where they’re back on Manhattan soil and need to push for the final 10 miles,” the founder of the Adonis app explained, saying that his pre-race morning routine each year is to “pop some bottles, shake off the night before, and make some last-minute signs.”

He’ll then navigate the crowds via scooter all the way from his Gramercy Park home base or whichever friend’s apartment he started his morning — with a step stool in tow so he doesn’t “need to push to the front every time” to see the runners, he said.

“Tens of thousands of people from all backgrounds suffering through a grueling athletic feat, no political tension, no winners and losers, just your fellow humans pushing you to the finish line,” Bobby Westside (second from right, with marathon companions) told The Post. Bobby Westside

Tom Fenninger, 32, is another big believer in the Queensboro Bridge spot.

“[That’s] arguably the most challenging part of the race — and when you get off, you just hear this roar of people lined up on First Avenue,” he told The Post. “At this point in the race, spectators have had their mimosas from brunch and are lit.”

Running more than two-dozen miles throughout NYC is as challenging physically as it is mentally, and the animated spectators help those thousands of runners greatly — maybe even more than they realize.

The NYC marathon is as big a day for runners as it is for spectators. Brian Zak/NY Post

Marathon spectators grab a selfie near The Sefton, a popular stop for the crowds. The Sefton

“For many, the cheers [from the spectators] drown out the negative voices,” Dr. Jonathan Jenkins, Psy.D., of Massachusetts General Hospital, told The Post.

“The more time a runner can spend in the moment, the fewer opportunities for anxiety and doubt to turn a great challenge into 26.2 miles of angst.”

Not every cue will help every runner, the expert noted, but he insisted that a joyful shout-out can bring a smile to any exhausted face — reminding the endurance testers that they’re loved by friends, and briefly, by strangers.

And tired, sweaty runners aren’t the only ones who benefit from rowdy cheering crowds — businesses along and near the marathon route do, too, because if the sidewalks are the orchestra, the bars are the chorus. 

Tim Hansberry, co-owner of Upper East Side pub The Sefton, said that marathon day is like no other for his business.

“People from all over New York, the U.S. — even all over the world — pass through,” he told The Post. 

The NYC bars, including The Sefton (above), are always bustling with lively crowds on marathon Sunday. The Sefton

The popular ringside spot at 1373 First Ave. stays slammed from 10 a.m. through the afternoon — Guinness being forever popular, with hard seltzers ascendant, Hansberry revealed.

Nearby on Second Avenue, The Supply House gets ready for the big day with a DJ spinning fun tunes and a full staff eagerly waiting to serve hundreds of breakfast burgers and craft beers to marathon-goers.

“[We see] Lots of families who have come to see their family members run early in the day. They grab some food and drink after watching their runner pass Mile 18 and then head over to the Central Park finish line,” Robbie Gillin, managing partner at The Supply House, at 1647 2nd Ave,, told The Post.

The Supply House sees crowds coming and going all day on the first Sunday in November. Supply House

Marathon finishers get special treatment at many of the NYC bars. Alex Williams

And of course, marathon runners get special VIP treatment:

“One tradition that we have [had] over the past 12 years is that when a ‘finisher’ walks through our front door draped in their marathon medal and their NY Marathon foil jacket, the whole place erupts in applause,” said Gillin. 

“And their first post-marathon drink is always on the house.”