On the morning of October 12, Eliud Kipchoge, the best marathoner on earth, will set off from the Reichsbrücke Bridge in Vienna, Austria, with the goal of traversing 26.2 miles—42.165 kilometers—in less than two hours, almost half a minute faster than any person in history.

A little less than 120 minutes later, the world will know whether he succeeded.

The sub-two-hour marathon is the last great barrier in distance running. Elite athletes have been closing in on the milestone for more than a century, but only in the past few years have they started to come close. Kipchoge holds the official world record of 2:01:39, which he set at the Berlin Marathon in 2018.

But as the running world (and its sponsors) have grown obsessed with breaking the two-hour barrier, they’ve sought to engineer the perfect race to finally see if a human could do it. And in that ideal-marathon bubble, Kipchoge has gotten even closer than 2:01:39.

In 2017, the Kenyan attempted to break the two-hour barrier at Italy’s Formula One race track, in Monza. He came agonizingly close, completing the race in 2:00:25, too slow by an average of less than one second per mile. An exhibition event sponsored by Nike, conditions in Monza had been so carefully controlled that the race was ineligible for world record consideration. Still, Kipchoge had come closer than many thought possible. As he said after the race: “The world now is just 25 seconds away.”

On Saturday, at 8:15 am local time (2:15 am ET), Kipchoge will make his second attempt at a sub-two-hour marathon at another exhibition event, this one organized by global manufacturing company Ineos. And while no one can predict the future, it looks like he has a solid chance of pulling it off. Conditions Saturday morning in Vienna look to be even more favorable than in Monza. Assuming he runs a smart, injury-free race, the following four factors will dictate Kipchoge’s success—or his failure.

The Shoes

The first time Kipchoge chased the sub-two-hour marathon, he did it wearing a special edition of Nike’s Vaporfly 4%, a shoe the company claimed could make runners 4 percent more efficient on their feet. In the past two years, peer-reviewed studies and analyses by WIRED and The New York Times have confirmed that, for many runners, the shoes perform as advertised.

But maybe not all runners. Some people respond less to the shoes than others. What’s more, 4 percent more efficient does not necessarily translate to 4 percent quicker. Some simple math confirms this: If the Vaporfly gave runners a 4 percent speed boost, every marathoner capable of running 2:05 or better (a rarified club, to be sure, but one with at least 10 members in 2019 alone) would need only slip on a pair to break two hours.

Yet since the shoes made their debut, several marathoners have posted shocking personal bests, the most recent example being Ethiopian Kenenisa Bekele, who last month won the Berlin Marathon in 2:01:41, a mere two seconds shy of Kipchoge’s world-best time.

Bekele ran that race in the latest version of Nike’s Vaporfly, a pair of shoes the company says saves runners even more energy than its predecessor. Kipchoge, meanwhile, has been spied practicing in Vienna in a Vaporfly prototype rumored to be more energy efficient than anything he’s worn since Monza.

man running and people trailing him

Kipchoge’s team of pacers could give him the edge he needs to clock in under 2:00. (They’ll be running in front of him, not behind.)

Photograph: INEOS 1:59 Challenge

The Pacers

One of the things that will make Kipchoge’s attempt in Vienna ineligible for official record consideration is the pacing strategy. In official marathons, pacers must start from the beginning of the race, and they can only shield the frontrunners from wind for as long as they’re physically able. The upshot is that pacers typically burn up and drop out long before the race is over, leaving competitors to run the last, most arduous miles unshielded.