Shalane Flanagan's feat of running marathons on back-to-back days is 'frankly inhuman,' according to an Olympic breakout star

• Former pro runner and 2017 NYC Marathon winner Shalane Flanagan ran marathons on back-to-back days.
• Olympic marathon breakout star and bronze medalist Molly Seidel says that is "frankly inhuman."

Shalane Flanagan is just days away from achieving the impossible: running each of the world's six major marathons in under three hours - all in 43 days.

To the common person, the feat seems otherworldly. Hell, even the top competitors in distance running, like marathon breakout star and Olympic bronze medalist Molly Seidel, see what she's doing as "frankly inhuman."

"It's literally incredible," Seidel told Insider. "It's been so fun getting to follow her from marathon to marathon. Obviously Shalane is one of the greatest ever, if not like the greatest American marathoner, and it's just so cool to get to see someone who is in her retirement but still striving for greatness and still absolutely kicking."

Flanagan - who won the 2017 NYC Marathon nine years after earning a silver medal in the 10,000 meters at the Beijing Olympics - retired from professional running in 2019. But after having "fallen back in love" with running, as she wrote in an Instagram post this September, Flanagan decided to step back into the spotlight for a feat of epic proportions.

The condensed marathon schedule this fall - Berlin, London, Chicago, Boston, Tokyo, and New York City in six weeks - presented a unique opportunity to check all of the world's major races off her bucket list in rapid succession. Perhaps the most ambitious, and certainly the most taxing, portion of that stretch came in the middle of October, when Flanagan conquered the Chicago and Boston marathons on back-to-back days.

"It's hard enough to double back three months after," Seidel said. "In my normal training, I'll occasionally have longer back-to-back days. But having to race like that, your body just aches after and your entire system is just so tired."

Not only did Flanagan complete both races with time to spare on her three-hour goal, but she actually bested her Chicago time by a full six minutes the following day in Boston despite carrying the fatigue of having run an entire 26.2 miles less than 24 hours prior.

Seidel marveled at the accomplishment.

"That is so incredibly difficult," she added. "Being able to like pick yourself up after 26.2 miles going super fast and be like, 'Okay, not only am I going to do this faster the next day, but I'm going to do it on a course that is so much harder.' Going from Chicago to Boston, like Boston is no joke of a course, so that was the most impressive part: running faster the next day on an even more difficult course."

Seidel knows a thing or two about succeeding on difficult terrain. The Team USA star shocked the world by winning a bronze medal at the Tokyo Olympics in just the third marathon of her career. She became the first American woman to reach the podium in the event since Deena Kastor finished third 17 years earlier in Athens.

Though her surprise success in Japan catapulted her into a whole new level of superstardom this summer, Seidel was previously well-known in running circles for her dominance at shorter distances during her time competing for the University of Notre Dame. Through it all, Seidel has cited Flanagan as a legend for whom she has "an enormous amount of respect."

"She's always been one of my heroes," Seidel said. "I got the chance to meet her when I was 18 years old. And I feel like she's been one of those people that's just been enormously influential and been such a good role model for the sport."

Both Flanagan and Seidel will line up in Staten Island in the wee hours of the morning Sunday for the New York City Marathon. Flanagan is poised to make history by completing the sixth and final marathon of her ambitious slate of majors this fall.

Seidel, meanwhile, says her goals for the iconic race through the Big Apple's five boroughs "are less place based or time based and more effort based," so she'll be content so long as she does "the absolute best that I can." And if enjoying the full experience is her top priority, surely seeing one of her heroes out on the course will help move the needle.

"I feel very lucky that I've gotten to have people like Shalane Flanagan to look up to," she added. "To be like, 'That's the kind of marathoner that I'm going to be.'"