Kristina Elfering, a 41-year-old civil engineer from North Oaks, Minn., felt ecstatic about running her first New York City Marathon to celebrate the 45th birthday of her friend and running mate, Victoria Nill.

But instead of crossing the finish line last weekend in Central Park after a successful 26.2 miles, Elfering collapsed at Mile 16, and nearly died.

“The first couple miles, I was feeling good,” Elfering tells The Post. “As we crossed the Queensboro Bridge, I looked at my watch and thought: ‘Wow, that was a really slow last mile.’”

Then the seasoned marathon runner started overheating.

“I remember coming off the bridge, [and going] around the corner — and then I don’t remember anything else.”

She collapsed, unconscious. The culprit: a clot in her left anterior descending artery — which made her heart stop beating, and her lungs stop breathing.

Little did she know, help was just behind her — giving her a second chance at life. And Friday, she met one of her saviors for the first time since the incident.

Staten Island-based physician, Dr. Theodore Strange, 59 — a doctor of internal and geriatric care who was running his 25th NYC Marathon with a 4½-hour goal — had just crossed the bridge after meeting with a friend, who gave him orange slices to eat.

“Any 10, 15 or 20 seconds could’ve changed the outcome of this,” says Strange, vice chairman of primary care at Northwell Health and vice president of medical operations at Staten Island University Hospital in Prince’s Bay.

He ran along the right side of East 59th Street and immediately knew something was awry.

“In the whir of the crowd — that’s one of the loudest areas [for supporters to cheer] — I just heard the word ‘Help!’ ”

Nill was standing over the unconscious Elfering. Initially, Strange assumed she’d passed out from dehydration.

“I realized very quickly something more was the matter,” he says. “She was turning blue and she was foaming at the mouth.”

And there was no pulse.

Strange quickly began CPR, and called out for a defibrillator, which the NYPD and EMS personnel quickly delivered. Three shocks elicited no response from Elfering’s heart.

“By the fourth defibrillation she came back,” says Strange. “She gasped and started breathing.”

The pros — Strange, NYPD officers, firefighters and EMS workers — then loaded a still-unconscious Elfering into an ambulance to New York-Presbyterian/Weill Cornell Medical Center — where her heart would be shocked once more.

All the while, Strange’s family — wife Valerie, 59; sons John, 29, and Marc, 27; and daughters Victoria, 25, and Elizabeth, 21 — were positioned on First Avenue, anxious to see him.

“When we saw there was a pause in the tracking system, we started to get a little nervous,” Valerie tells The Post. “I said: ‘Gee, why is he taking so long? Is there something wrong with my phone?’ ” adding on that day, her husband hadn’t gone out with his inhaler.

When he finally approached — after spending some 20 to 30 minutes at the scene with Elfering and the emergency officials — Valerie knew something was off.

“He wasn’t himself — he was dragging,” she says. “He walked over and just broke down, ‘I just had to give someone CPR.’ ”

And Strange himself, shaken from the experience, didn’t know if he could finish the race.

“It’s OK! It’s OK!” Valerie recalls telling him. “You did your job. Get on with it! Do it for [Elfering!]”

Strange crossed the finish line with a time of 5 hours, 16 minutes.

“I had no idea when I finished the race what was going on,” says Strange, adding that through a family EMS contact, he found out Elfering was in the emergency room, alive and stable.

In the coming days, through a friend of a colleague whose girlfriend was Elfering’s nurse, Strange was told her family wanted to meet him.

On Friday, they finally did — with both referring to the meeting as emotional.

“They wanted to know who their hero was,” Strange says. But he doesn’t consider himself a hero.

“I was a tool in this,” he says. “I don’t consider this heroic . . . I consider it my duty and my obligation. Let me give kudos to NYPD, FDNY and EMS because I didn’t do this alone. It was a well-orchestrated team approach that got her through this.”

Elfering later received a stent in that artery to keep it open for bloodflow. She’s now wearing a defibrillator vest for at least two months — and it’s too early to tell whether she’ll need a defibrillator surgically implanted.

“She’s going to survive,” says Strange.

That’s also a clear priority for Elfering.

“I’m hoping to run the [New York City] Marathon next year if they let me in,” she says, adding she aims to run it alongside Strange. “I have unfinished business.”