Long before Ser-Od Bat-Ochir became one of the most prolific distance runners in the world, he planted himself on the start line of the Hong Kong Marathon in 2002. At the time, Ser-Od had never run anything longer than 20 kilometers — or about 12 miles — even in training.

“I didn’t know what I was doing,” he said.

That hardly prevented him from running with a lead group of Kenyans for the first few miles, after which the marathon imposed its remorseless brand of agony. As he labored to the finish line, well out of contention, Ser-Od came to an important realization: Marathons are long and difficult.

“I just thought, I don’t want to do this again,” he said. “But here I am.”

Yes, here is Ser-Od, now 41, and there is no one else quite like him. A five-time Olympian, he has now run in 74 marathons and represented Mongolia at every major international competition since 2003.

On Sunday morning, with the support of his wife, Oyuntuya Odonsuren, who moonlights as his coach, Ser-Od will make his 11th straight appearance at the World Athletics Championships when he tackles the streets of Budapest in the men’s marathon.

In the process, Ser-Od has become a uniquely popular figure in the marathon world: a self-made runner who emerged from obscurity to become a near-permanent presence on the global stage.

“Tough as nails,” said Tim Hutchings, a broadcaster and former world-class runner, “and a gentle, smiling soul.”

Ser-Od, whose 5-foot-7 frame has the smooth aerodynamics of a hang glider, still has outsize goals. He hopes to improve on his personal best of 2 hours 8 minutes 50 seconds. He hopes to place among the top eight at a major marathon. And he hopes to race next summer at the Paris Olympics.

“I know it won’t be easy,” he said.

But when has his path ever been easy? In an interview over coffee on a recent afternoon, he thought back to his roots, recalling his childhood in Ulaanbaatar, the capital of Mongolia, where his father taught industrial arts and his mother was a kindergarten teacher.

Ser-Od was not particularly academic-minded when he was young — “There was nothing I hated more than studying,” he said, laughing — but he was a good athlete. His first race was at a school sports festival, where he and his classmates were given five minutes to see how far they could run. Ser-Od won easily.

“I loved that feeling,” he said in Japanese through his agent, Brett Larner, who also acted as his translator.

Ser-Od continued to run throughout high school and, after attending university, briefly taught physical education. But the pay was meager, he said, and the long hours cut into his training. He often had no choice but to run at night, and if you’ve never experienced the splendors of jogging on a chilly evening in Mongolia, Ser-Od can tell you all about it.

“It gets quite cold and dark,” he said.

Back when Ser-Od was starting out, Mongolia lacked much of a running culture, he said. People would see him bundled up in four or five layers of sweats and stare at him as if he were juggling cats on a unicycle.

But he was already dreaming big, having watched on television as Haile Gebrselassie of Ethiopia won the men’s 10,000 meters at the 2000 Summer Olympics. Ser-Od began to wonder: How does one go about becoming an international athlete? Would it be possible for him to compete at the world championships? Or even at the Olympics?

“And just because there’s not really any history of athletics or running in Mongolia, nobody knew,” he said. “It was a learning process.”

After his marathon debut in Hong Kong, Ser-Od quit his teaching job and joined the national police as an officer, one who could win races. The national police had a track and field club, and Ser-Od was a bit of a ringer.

More important, Ser-Od now had the requisite funding to train on more of a regular basis. In 2003, he made his first appearance at the world championships, placing 63rd in a time of 2:26.39, which demolished Mongolia’s national record by about 10 minutes.

“Everybody was just amazed that a Mongolian could run that fast,” Ser-Od said. “They said that it was crazy, that nobody would ever break it.”

Ser-Od continued to break it — he ran a test event for the 2008 Olympic marathon in 2:14.15 — but he was confident that he still had untapped potential when, a year later, he met Gebrselassie at a road race in England. Ser-Od said he was able to dine with Gebrselassie a couple of times and took full advantage of the opportunity to pepper him with questions about training.

“I still didn’t know what I was doing,” Ser-Od said. “So I asked him, ‘What does a world-class marathoner need to do to run at that level?’ And Haile said, ‘The most important thing is to identify what works for you and don’t worry about what others are doing.’”

After the race, Ser-Od was getting off an elevator when he bumped into Gebrselassie again.

“And I’ll never forget this: He asked if we could get a picture together,” Ser-Od said.

It was a formative moment for Ser-Od, who drew inspiration from their encounter and continued to improve. He broke through with a top-10 finish at the 2011 London Marathon. What was working for him? A grueling training program that seemed to invite all of the planet’s atmospheric conditions.

“I was training completely by myself, and I was doing it all,” he said. “I was training in the heat. I was training in the snow. I was training in the rain. I was training in the dark. And that produced results.”

It was also taking a toll. By 2014, Ser-Od knew that he could use some company — “Training by yourself is really draining,” he said — so he moved with his wife and four children to Japan, where he joined a professional team.

But marathoning is an unforgiving profession, and when Ser-Od found himself without a sponsor after the Tokyo Olympics in 2021, he fell into a funk. He thought his career was finished. He reached out to Larner, whom he had met through running circles.

“I was like, ‘Uh, I’m a big fan, but a 40-year-old Mongolian? How am I going to find you a sponsor?’” Larner recalled. “I told him I’d see what I could do, but I thought it was pretty hopeless.”

After making several inquiries that went nowhere, Larner was connected with Shingo Oshiro, the president of a solar panel company that had recently started a women’s running team. Oshiro offered Ser-Od a contract and told him he would hire him as a coach for the team once he retired from racing.

“I was so appreciative that they believed in this idea of going for a sixth Olympics and wanted to support me,” Ser-Od said. “I really want to repay my debt to them.”

Still, he knows that making it to the Paris Games next year will be another challenge. He is, in some ways, a victim of his own success. It is all relative, but marathoning in Mongolia has become more popular thanks in part to Ser-Od. He recalled visiting Ulaanbaatar this spring — he still has a home there — and getting stopped for selfies.

“Oh, it’s Ser-Od!” he recalled people shouting.

In a development that would have been unthinkable a few years ago, there are now four Mongolian men who are competitive enough to race at events like the world championships. The problem is that the country can send only three of them to major international competitions.

In fact, Ser-Od thought he was in danger of missing out on Budapest. After he placed 26th at last year’s world championships in Eugene, Ore., injuries hindered his training. As a result, his national ranking slipped to fourth. After an unspectacular result at the Copenhagen Marathon in May, he braced himself for the worst.

“We kind of thought, Eh, that’s probably it,” Larner said. “But there was a miracle.”

It turned out that one of Ser-Od’s Mongolian rivals had raced poorly in Copenhagen. The country’s athletics federation subsequently awarded its final spot in the world championships to Ser-Od.

“It was lucky,” Larner said. “Very lucky.”

Of course, there is nothing wrong with a little luck, especially after so many years of hard work. Against all odds, Ser-Od’s finish still seems far away.