Mile 15 is famously the loneliest stretch of the New York City Marathon, where runners trade the rowdy fan-packed streets of the boroughs for the quiet mile-long Queensboro Bridge as fatigue sets in.

But there was a massive party right around the corner Sunday.

When this year’s event featuring a record-breaking pack of about 60,000 racers turned onto Manhattan’s First Avenue, they were rewarded with the loudest and most fervent cheers of the whole race as fans clogged the sidewalks of the bar and restaurant-filled artery.

Among the throngs of fanatical supporters packing the Upper East Side bash were Alexa Little, 33, of Charlotte, North Carolina, and others clad in pink outfits to cheer on friend Emily Kellog, a 29-year-old with stage-four breast cancer.

“She trained all through her chemotherapy in order to run here today,” Little said.

“This is her first marathon, and she’s running it with her sister who’s also running for the first time. The whole family will be here wearing pink about 20 of them. She’s incredibly determined. This is her dream.”

Also along the stretch were four Delaware high-school track stars who woke up at 3 a.m. to surprise their coach, Katrina Holloway, 50.

“She doesn’t know we’re here, but she’s definitely going to spot us,” said Sophia Harrison, 18 of Conrad Schools of Science in Wilmington.

Harrison was hoisting a sign that featured signatures from all the members of the cross country team coached by Holloway, who was running the race to raise money for cystic fibrosis.

“We wrote her name real big on this sign, and we’re going to yell all the inspirational things she’s been yelling to us during our Champions season this year,” the teen said.

“We’re going to yell. ‘Trust your training! Gumption!’”

Holloway’s trek of the five boroughs was just one of the ways the beloved teacher motivates her students.

“She’s our English teacher and the coach for all of our running teams. The cross country team. The track team. She inspires us,” Harrison said.

A pair of Manhattan women deployed an eye-catching tactic to cheer on their favorite runner.

Upper West Side physical therapist Cristina Fuentes, 29, and her soon-to-be mother-in-law Lane Pandrok, 59, held up sticks with professional print-outs of Jake Pandrok, 28.

“Jake has six family members coming today, three of them from Arizona,” Fuentes said of her fiancé.

“They’ve been coming to this spot, it’s kind of like a family spot when anyone runs the marathon in the family. It’s a great spot. And the [print-out] heads are just funny, and they work!”

Jake’s cheering section also donned shirts reading “Run Jake, Run! New York Marathon 2026,” as they cheered the competitors turning off the Queensboro Bridge.

“This spot is also great because the energy from the runners coming off the bridge right here is just incredible,” Fuentes said.

A group of fans from Germany also stood out by wearing red arrows over their heads to direct runners to the finish line.

Anja Wahner, a real-estate broker from Berlin, came with her brother Christian Wahner, 42 and his girlfriend Wenke Nieswandt, 40, both from Hamburg, to cheer on Anja’s boyfriend running his first Big Apple marathon.

“We bought these [arrows] in Hamburg,” Anja said. “No one here has ever seen this before. Everyone is smiling at us and pointing and asking, ‘Where did you get this? How did you think this up?’ ”

After soaking in the frenetic First Avenue scene, runners made their way up to The Bronx before coming back down into Central Park for the race’s famous final push.