Claire Hallissey, right, with Freya Murray, celebrate their finish at the London Marathon. Hallissey’s finish in 2 hours 27 minutes 44 seconds earned her the opportunity to compete in the Summer Games in London. (SUZANNE PLUNKETT/REUTERS)

It was all so improbable. Arlington’s Claire Hallissey refused to allow herself to think of making the British Olympic team during the final miles of the London Marathon on April 22, even though she led her British rivals, even though she was on pace to hit the qualifying time, and even though the enthusiastic crowd lining the course kept reminding her of what she was chasing.

She felt no one nearby, but feared a rival would appear on her shoulder at any minute. She worried she would place first among the British women, but not fast enough to claim the third and final remaining Olympic slot. She feared she would break down, or stumble, over the last mile or two, missing an opportunity that seemed so close.

“Once I got to 16 miles, it just became a battle,” said Hallissey, who was born, raised and educated in England. “I turned a corner with like 200 meters to go, and I heard the announcer say, ‘You can see the pain on her face!’ At that point, it forced me to relax a little bit and enjoy the moment . . . I was just so conscious that it wasn’t over until it was over.”

When Hallissey, 29, finally crossed the finish line in 11th place overall and first among her countrywomen, she wore a huge, incredulous smile. Her finish in 2 hours 27 minutes 44 seconds earned her the opportunity to join Paula Radcliffe, the world-record holder, and Mara Yamauchi on Britain’s Olympic women’s marathon team, which will compete at the Summer Games in London.

From their home in Arlington, her husband Matt Hallissey, also British, followed his wife’s results in the wee hours of the morning on the Internet.

“She had been making steady progress for the last six or seven years . . . [but] this was really a distant dream,” said Hallissey, a project manager for the international company AECOM. “She had put the work it, but it was one race, all or nothing.”

It was also only the third marathon Hallissey had ever run, leading to depictions of her in the British media as an out-of-nowhere star. To clinch the spot, she not only had to top the British field, but also surpass the time of 2:28:24 that Jo Pavey ran at the same race last year. Pavey decided to sit out the event, banking on the hope that the time would hold up and secure her the third Olympic berth. Hallissey, however, foiled that plan.

“I knew we were on [pace] for the time,” Hallissey said. “I was pushing as hard as I thought was safe to push it to make sure I had enough to get to the finish.”

Hallissey, whose husband accepted his Virginia-based position in 2009, joined him in 2010 after finishing her doctorate in mucosal immunology at the University of Bristol. She had run mostly middle distances as a school girl in Watford, England. A strong athlete, and very fit, Hallissey excelled but fell short of stardom. It did not cross her mind that she would someday compete at the Olympic Games.

“As a club-level runner, I would occasionally compete” at city competitions, she said. “I was never remotely near getting an international appearance in the junior ranks.”

Back in 2005, when London was awarded the 2012 Summer Games, Hallissey trained with and occasionally competed for running clubs, but it was hardly serious business. She did, though, enjoy challenging herself. And running provided a break from her heady research into meningitis.

As she moved beyond the 800- and 1,500-meter races she ran in her youth, attempting 3,000 and 5,000 races on the track, then 5K and 10K road races, she found surprising gratification: She did better, relatively speaking, as the races grew longer.

A year after running her first 10K road race in 2004, she decided to try a half-marathon. In the 2005 Grunty Fen Half Marathon just north of Cambridge, where she earned her undegraduate degree, she finished second in 85:04.

“It was more just a natural progression as I worked my way up through the distances,” Hallissey said. “As the races got longer, I did comparatively better in relation to other people.”

After turning in her thesis and settling in Virginia, Hallissey decided to run the 2010 Cherry Blossom 10-miler here in Washington, her first road race in the United States. She finished eighth in 54:57. She ran a host of other small races throughout the region, and won many of them, including the Capital Crescent 5k in Bethesda (16:46), the Jingle All the Way 10K in Washington (35:17) and the Leesburg 10K (33:54).

She met George Buckheit, the coach at the Capital Area Runners training group, at an event, and joined his club. She logged laps around the track at Washington-Lee High in Arlington and went on long runs along the C&O Canal.

In November, Hallissey decided to tackle her first marathon, wondering if her propensity for better performances at longer distances would extend to the grandest road race of all. She finished a respectable 18th in the New York City Marathon in 2:36.13.

It was then that she and her husband began pondering her Olympic prospects.

“That was a very, very good time,” Matt Hallissey said. “That’s when it kind of became possible.”

The next year, she ran the Chicago marathon. Her finish in 2:29:27 convinced her she was seriously in the mix for an Olympic spot. With her doctoral research out of the way, she could put all of her attention into that new goal.

“I’ve wondered what [her] limit would be,” Matt Hallissey said. “There’s hasn’t been one yet.”