With the world’s six major marathons — Berlin, London, Chicago, Boston, Tokyo and New York City — squeezed into a six-week window this fall, most top runners had a tough decision to make.

Then there was Shalane Flanagan.

Flanagan, who won the 2017 New York City Marathon, these days coaches Nike’s Bowerman Track Club in Portland, Ore. But she saw an opportunity in the closely packed schedule created by the coronavirus pandemic, which pushed three spring races into the fall. Most marathoners wanted to run just one marathon. Flanagan wanted to run in all six, and to try to complete each one in under three hours, a pace of under 6 minutes 50 seconds per mile.

So far, so good. She ran the Berlin Marathon on Sept. 26 in a faster-than-expected 2 hours 38 minutes. Seven days later, she finished in London in 2:35:04.

Now comes an exhausting holiday weekend: the Chicago Marathon on Sunday, followed by the Boston Marathon on Monday. That is two marathons, nearly a thousand miles apart, in roughly 28 hours.

If she can walk after that, she will do a virtual version of the Tokyo Marathon at home in Oregon next weekend, since organizers canceled the in-person event because of the pandemic. Then it’s off to the New York City Marathon, which is Nov. 7.

That’s a heavy workload after two major knee reconstructions in 2019. Her patellas have hamstring tendons from cadavers.

“I missed pushing myself,” Flanagan, 40, said of life after the end of her competitive running career. “It was just fun to have a big goal again.”

So much for taking it easy in retirement.

As it turns out, old habits, especially for people who have spent the vast majority of their lives competing and whose identities are so closely tied to running races, die very hard.

Deena Kastor, 48, the American record-holder in the marathon, maintains she will never officially retire and is planning to race in the Berlin marathon next year. Kara Goucher, 43, an Olympian in 2008 and 2012, has in recent years competed in trail races, and doesn’t plan to stop any time soon.

“We all reach a point where we know we can’t make that podium anymore, but it’s difficult at that point to just walk away and not challenge yourself anymore,” Goucher said this week.

Still, six sub-three-hour marathons in 43 days is a particularly gritty challenge. It would gain Flanagan, who rarely ran more than two marathons a year in her professional career, entry into the Marathon Maniacs, a group in Tacoma, Wash., that requires aspiring members to run two marathons within 16 days or three within 90.

And yet, two sub-three-hour marathons in 28 hours, with the second one being the hardest of the six majors because of the dreaded Newton hills in the second half of the race, is on a different level of kooky.

“It’s so typical of Boston to be the super hard part,” said Flanagan, who grew up in Marblehead, Mass., northeast of the city.

This weekend’s double has loomed over her training. She hunted for training spots in sweltering Tokyo during the Olympics, where several of the athletes she coaches were competing. She was not allowed outside the Olympic bubble, though she could run on the warm-up track with her athletes outside the Olympic Stadium — until it was clogged with hurdlers and sprinters. The U.S. team’s training facility was a 45-minute bus ride away. She missed several days of training, and never had a run that lasted more than 10 miles while in Japan.

She tried to mimic a shorter version of the Chicago-Boston double last month, running 20-plus miles on a flat course one day, then 21 miles at a 6:40 per mile pace on hilly terrain the next day. Changing her 17-month-old son’s diapers and working in her garden after the first run served as a stand-in for what could be a hectic journey from Chicago to Boston.

In Berlin, where she started conservatively, Flanagan averaged about 6:40 per mile for the first third of the race as she ran with her physical therapist, Colleen Little, a sub-three-hour marathoner in her own right. But on Berlin’s flat course on a not-too-sunny day, she hit the gas in the second half, clicking off a 5:30 mile at one point.

Speaking from London days later, she said her back was sore, probably from lugging her son around. Otherwise she was fine. Still, she said, she was planning to slow down for London.

She didn’t.

She landed in a corral with the sub-elite men, got caught in their wave of speed, lost track of her splits and ended up passing the halfway mark in 75 minutes. Whoops.

She told herself it was time to back off, but barely did, even though she knew the door to the pain cave might be just around the corner. Her body started to quit around the 20-mile mark. Her quads were on fire, and she could barely lift her legs. She even started walking in the last mile because she thought she was going to fall over. She finished in just over 2:35.

“That last five kilometers was absolutely brutal,” she said on Tuesday.

Her legs had started to recover, she said, but her right foot was a bit janky. It felt as if she had rolled her right ankle even though she knew she hadn’t.

On Wednesday, Little said, a massage and some joint mobilization work helped matters, especially with the sore ankle.

“When Shalane told me she wanted to do this, the physical therapist in me was wary but the endurance athlete in me was like, ‘I want to do it with you,’” Little said. “I just want her to be as healthy coming out of this as she was going in.”

Flanagan is sure she can be, and she really does plan to slow down this weekend so she can accomplish the original goal of completing all six races.

“I know I am a better person if I run,” she said. “I just needed something else other than running for the sake of running.”