Until about a decade ago, any talk of a human finishing a marathon in less than two hours — especially on a more challenging course, like Boston’s — was largely written off as a pipe dream.

But then elite marathoner Eliud Kipchoge emerged on the global stage, breezing through long-held records and stunning the world by redefining the human body’s capacity for endurance. Kipchoge is scheduled to run the Boston Marathon for the first time this weekend, begging the question: what would it take to crack the elusive, 120-minute time code on this iconic course?

The GOAT’s debut in the Boston Marathon

The 38-year-old Kipchoge is often heralded as the Greatest of All Time in the marathon: He holds two Olympic gold medals in the event, four first-place finishes at the London Marathon, another four at the Berlin Marathon, a first-place finish in Tokyo and one more in Chicago, for good measure.

He also holds the marathon world record, with the time of 2:01:09, which he ran in Berlin last year.

“My plan is not really to run a course record or anything else, but my plan is I want to see myself winning,” Kipchoge told reporters after he announced he would run Boston last year.

But of all of Kipchoge’s extraordinary accomplishments, there’s one that stands alone: In 2019, Kipchoge ran a marathon in under two hours at the Ineos 1:59 challenge, a controlled event in Vienna, Austria, with a time of 1:59:40.2, becoming the first person to ever run a marathon in less than two hours.

But that time didn’t count as a world record for several reasons: There was no competition, Kipchoge used a special team of pacemakers and he had hydration delivered from a bike, among other factors.

But it gave the first glimpse ever into the possibility of humans being able to break into running’s next frontier.

Mark Carroll, head coach of the Boston Athletic Association’s High Performance Team, believes we’re close to a sub-two-hour marathon occurring in an official setting.

“Can it be done? [Kipchoge’s] done it. He’s done it on a closed course with pacemakers. He’s shown that sub-two-hours is possible in a controlled setting,” he said. “But I think the day where we see a two-hour marathon in a world marathon major is probably not too far away.”

Why Boston’s course is among the most daunting

With its sloping terrain and temperamental weather, Bostonmay be the hardest of the six Abbott World Marathon Majors to conquer.

The course starts downhill, but around the 16 mile mark the elevation kicks runners right in the face as the route merges into Newton and the notorious “Heartbreak Hill.”

Combine that with New England’s mercurial spring weather, with race day conditions varying from a freezing cold downpour in 2018 to temperatures of nearly 90 degrees in 2012, and it’s anyone’s guess what runners will face.

Nathan Cardoos, medical director at the Ryan Center for Sports Medicine at Boston University, predicts that breaking two hours will happen in a lot of other places before it happens here.

“It’s not gonna be in Boston where it happens the first time,” he said.

The math and science behind a marathon performance

To run a sub-two-hour marathon, Carroll calculates that a runner would have to run about a 4:40 mile for just over 26 miles straight. To do that, their body will have to be exceptionally pristine.

Cardoos says that most elite marathoners simply have a bigger "engine" than other people. Measured by what’s called V02 max, Cardoos explained it as an equation that counts the amount of blood you pump through your body times the amount of oxygen that’s being extracted by the tissue. Most people have a V02 max around 35 or 40.

"These elite marathoners will have V02 max in the 70s or in the 80s. So, the size of their 'engine' is almost twice what a normal, untrained adult male would be," he said.

Another factor is lactate threshold, which measures when lactate buildup in the blood stream during exercise is higher than resting levels. A third factor is running economy, which essentially measures how effeciently runners use oxygen and energy.

Cardoos points to a 1991 research paper that hypothesized that the maximum human limit for a marathon was right around an hour and 58 minutes.

Given how far our understanding of running has come, Cardoos thinks that could be achieved within the next 10 to 20 years.

“So in the marathon, in the last 20 years, I think there have been eight world records set. So, there’s been considerable advancement in the marathon times over the last 20 years. And I would expect that that would continue to improve,” Cardoos said.

So why have marathoners gotten so much faster?

Understanding why humans have gotten faster at marathon running could hold part of the answer to breaking two hours.

In the year 490 BCE, according to legend, a runner named Pheidippides was sent on foot to carry word of Athenian victory over the Persians at the Battle of Marathon. As the tale goes, Pheidippides — who had run for multiple days as a messenger — hoofed the roughly 25 miles to Athens, greeting the city with the triumphant news of their army’s victory. He then promptly died.

A black-and-white engraving shows a man in ancient Grecian clothing collapsed on the floor next to two other people who are checking to see if he is still alive.
Greek soldier Pheidippides arrives in Athens with news of the Greek victory over the Persians at Marathon, only to die on the spot, 490 BC. An engraving by C. Roberts after Frederick George Cotman.
Hulton Archive Getty Images

Obviously, the sport of running has come a long way since then, with better shoes, full-time coaches and high-tech training apps.

In 1896, at the first modern Olympic Summer Games, Greek runner Spyridon Louis won the marathon in Athens with a time of 2:58:50, becoming the first Olympic marathon champion. (That race was run at a distance of 24.8 miles.) Since then, elite runners have cut nearly an hour from that groundbreaking run.

But it’s not that people are necessarily getting inherently faster. They just understand the science of running more.

“You know, any team now in the NFL would absolutely crush the Miami Dolphins from [1972] that won every game for the whole season, they’re just better now,” Cardoos said. “And the human physiology hasn’t changed that much to explain that.”

More appreciation for the mental side of sports is also a factor.

The B.A.A.'s Carroll said that along with all of the physical traits, having a sharp mind is a necessity for anyone who wants to run under the hallowed two-hour mark.

“You know, it’s gonna take an athlete that can endure a lot and can hurt for a long time. That’s the x-factor, really, when it comes to marathoning,” he said. “We all know it’s gonna hurt, right? And some folks are able to deal with that pain. You’re putting your body through a lot of pain and torture there in the last 30, 40 minutes in particular.”

Leonard Zaichkowsky, a sports scientist who is a pioneer in the field of sports psychology, said that beyond just being able to deal with pain, runners who win marathons think a little bit differently than the average elite athlete.

“They’re cognitively sharp, they make smart decisions during the course of the run, they know when to accelerate, when to decelerate, they’re just a little smarter than the competitive field, in my opinion,” he said.

"It's gonna take an athlete that can endure a lot and can hurt for a long time. That's the x-factor, really, when it comes to marathoning."
Mark Carroll, director of performance and head coach of the Boston Athletic Association’s High Performance Team

According to Zaichkowsky, there are two strategies for dealing with the enormous stress they’re putting on their bodies: One is to associate directly with the pain and work their way through it, or they disassociate entirely, imagining themselves as something like a steam engine or tractor to push through. Some runners switch back and forth.

Zaichkowsky is an advocate of cognitive training, an emerging field that aims to find ways to develop the mind of athletes similar to the way the body is trained. He thinks that it’s possible to get under two hours in a formal marathon training, but to do so, runners will have to adapt the training. Even then, he thinks we’re “probably pretty darn close” to the limits of human runners in a marathon.

“There are limits to everything,” he said. “The human body and limits to the brain’s capacity to tolerate the kind of pain that accompanies something as challenging as a marathon.”

'The greatest feat in human endurance'

Part of the reason experts think a sub-two-hour marathon is possible is because what’s considered impossible in running has changed. Before British neurologist Roger Bannister did it in 1954, the general consensus was that running the mile in under four minutes wasn’t possible.

Carroll pointed out that 20 years ago, people weren’t running the marathon under 2:05 on the men’s side, and 2:20 was considered almost impossible for women. While running under those times is still a major achievement, it’s becoming more and more common.

Better training, better shoe technology and a better understanding of nutrition and the human body have all set up the opportunity to pull off what could be the greatest feat in the history of the sport in competition.

“I would say a sub-two-hour marathon is probably one of the greatest, if not the greatest, feat in human endurance, really,” Carroll said.

While Boston may not have a two-hour time in its cards anytime soon, people like Eliud Kipchoge have shown that human limits can be flexible. After he ran under two hours in 2019, he talked to reporters about his motivation.

“I’ve been putting in my heart and my mind that I want to run under two hours in [the] marathon to make history and pass a positive thought and a message [to] the whole world that no human is limited,” he said.