Eliud Kipchoge is about to attempt something no human has ever done before; he is going to try to run the length of a marathon in under two hours.
Yet should the 34-year-old accomplish this feat of human endurance, he won't make it into the record books.
It's the Kenyan himself who holds the current World Record - a blistering 2.01.39, set at the Berlin Marathon in September 2018.
But he's run faster before.
Kipchoge clocked an impressive 2.00.25 in a previous attempt to break two hours in Monza, Italy, in May 2017, but fell short of one of the last remaining barriers in athletics.
On Saturday (or in the following eight days if the conditions are not ideal) Kipchoge is going to try to achieve what he was unable to before.
The Ineos 1:59 Challenge will see the runner take to the Prater, a park in Vienna.
He'll run a flat 5.97 mile loop with no gradient 4.4 times (a total of 26.2 miles) in a bid to make history.
So why, if Kipchoge does it, will it not be a World Record?
For a race to meet athletics' competition rules, and therefore be eligible to become a World Record, certain criteria must be met.
Yannis Nikolaou of the International Association of Athletics Federation (IAAF) - athletics' governing body - told ITV News that races must:
Be sanctioned by the IAAF or by the Athletics Federation of the host country.
The course must be measured and certified by an official.
There must be anti-doping controls.
There must be at least three competitors.
Pacemakers cannot rotate in and out of a race.
An athlete cannot be paced by a vehicle with a motor.
Refreshments must be taken from official stations and not brought to an athlete.
During Kipchoge's first attempt in Italy in May 2017, refreshments were brought to him and pacers swapped in and out, meaning the run fell foul of the IAAF's rules.
Again in Vienna, pacemakers running just ahead of Kipchoge will swap in and out, but crucially the Kenyan is the only athlete attempting the challenge.
Refreshments in the form of eight pre-prepared drinks, energy and caffeine gels will also be handed to him every 3.1 miles (5km), breaching the rules.
So why do it if it won't be classed as a World Record?
"It's an attempt to break barriers," Mara Yamauchi told ITV News.
The former Olympic marathon runner is the second fastest British woman behind World Record holder Paula Radcliffe.
"The two hour marathon is one of the final barriers left in athletics."
Sir Roger Bannister's sub-four minute mile, set in 1954, has long since been broken - Morocco's Hicham El Guerrouj, the current World Record holder, is now well under it with a time of 3.43.13 - but people still remember the name.
All it takes is 'executing the perfect race'
Yamauchi - whose personal best is 2.23.12 - believes Kipchoge can pull off the sub-two hour marathon.
"Kipchoge has shown he has the mental strength," Yamauchi who came sixth in the Beijing Olympics says.
"He's set the world record and won the London Marathon four times.
"He has been a world-class athlete for a long time."
Success will be a combination of both physical and mental factors she believes.
She adds that "no stone will have been left unturned" by his team in their quest for victory.
Speaking from experience, she told ITV News: "If you're very well prepared, which Kipchoge will be, it's not easy, but it's do-able.
"In the first half of a marathon, because you're running well below your maximum speed, it doesn't feel too hard, but after 16 or 17 miles the race begins to get really tough as you're starting to tire, but you're still very far from the end, so you have to draw on your reserves and mental strength to stay focused."
In short: "It all boils down to executing the perfect race, and physically executing the plan as optimally as possible."
So just what is the perfect plan?
Kipchoge has a dedicated team of six behind him, all focused on helping him to achieve the seemingly impossible.
When Kipchoge first attempted Nike's Breaking2 in 2017, he ran a phenomenal time of 2.00.25, so all he needs to do is drop 26 seconds, just one second per mile, to break the two hour barrier.
But when you're running at your absolute maximum, an extra second per mile is some ask.
So, how is he going to shave off the seconds?
Training for the big day
Professor David James, a professor of exercise science at the University of Gloucestershire, believes "it's definitely possible that Kicphoge can run a sub-two hour marathon.
"There's no reason why not and he has come very close already."
Prof James points out that all Kipchoge's training has been focused towards the Ineos 1:59 Challenge since he set a course record of 2.02.37 at the London Marathon in April.
As well as high-intensity interval sessions and long runs of slower miles, Kipchoge's coach, Patrick Sang, has also incorporated more strength sessions into his routine, says Prof James.
These he believes are essential to improve running form and help to "sustain running economy" (running efficiently) which is most beneficial in the latter stages of the race when he will begin to tire.
He has also upped his mileage from his pre-London Marathon training, from 118-130 miles per week to between 124 and 140 miles.
For Kipchoge it's all about visualising his goal
While Kipchoge's entire team and pacers are listed on the Ineos 1:59 Challenge website, there is no mention of a psychologist.
With psychology so important for athletes hoping to win anything, Dr Josephine Perry, Sport Psychologist and author of Performing under Pressure said it is "surprising" that he does not appear to have a psychologist.
"When athletes are so close to achieving something, it is often the psychology holding the back," Dr Perry says.
And when all that's holding Kipchoge back is less than 26 seconds, it would seem strange that he does not appear to have a dedicated psychologist.
Earlier this year, when Kipchoge announced he would be running the challenge, he repeatedly talked about the mindset required to break two hours.
"I have visualised it," the 34-year-old told the assembled press.
"I have put it in my heart and my mind that I will break the two-hour barrier.
"Breaking the two-hour barrier is crucial for me."
Kipchoge himself has put psychology at the heart of what he will do, stating: "My mental preparation for taking on such as challenge is just as important as my physical preparation."
"He's doing four laps" of the Prater, Dr Perry notes, "so when he's coming to the end of the second lap and the beginning of the third, it's going to be very tough mentally, and he'll be at the limits of what he can do."
It is then she believes that Kipchoge will employ "imagery" or the "visualisation" that Kipchoge mentioned.
Imagery involves athletes recreating events mentally - even to the point of creating scripts and imagining how their hands will feel, how dry their mouth might feel - so that they can prepare themselves for the challenges they will face.
In the run-up to the race, Kipchoge cannot accurately experience the toughness of the 22-mile mark.
Instead he must mentally create memories to trick his brain into feeling that he has already done this and overcome the exact challenge before, explains Dr Perry.
Why pacers could prove essential
Prof James highlights the importance of pacers in the run.
A group will run in a "V" formation ahead of Kipchoge, not only setting his pace, but easing any wind resistance on him and allowing the Kenyan to run in their slipstream.
While this might not seem much, a canonical study by Griffith Pugh in 1971 found that to run at a pace of one mile every four-and-a-half minutes, drafting one metre behind another runner on a still day saves about 80% of the energy you'd otherwise spend fighting air resistance.
That corresponds to about one second per 400 metres at that pace, and more on windy days.
The minimum time Kipchoge can run a mile in to break two hours
The minimum time Kipchoge can run a kilometre in to break two hours
Over a marathon distance, this would theoretically save 105.5 seconds (one minute and 45.5 seconds).
Kipchoge's current fastest marathon record in which he did not have pacers in whose slipstream he could run was in Berlin in September 2018, where he set a blistering course record of 2.01.39.
To break two hours, Kipchoge needs to be running at a minimum of a 4.34.5 mile or 2.50km pace, meaning he will not be far off the time used in Pugh's calculations.
So even if he could save 40 seconds, he would be under two hours.
Prof James estimates Kipchoge's pace will be set at achieving the upper-end of a 1.58 marathon, giving him time to play with should he tire towards the end or not quite meet his pace throughout, meaning there is less chance the athlete will miss out on his goal by a matter of seconds again.
This pace will be carefully controlled by a vehicle travelling ahead of the pace runners and shining a laser onto the ground which they need to be running just behind.
Using the impact of 250,000 cheering supporters to break records
The biggest departure the Ineos 1:59 Challenge is making from Nike's Breaking2 is the crowd.
When Kipchoge, along with Zersenay Tadese of Eritrea and Lelisa Desisa of Ethiopia, embarked on 42.195 km around Autodromo Nazionale Monza, it was closed to crowds.
This time organisers hope to line the route with 250,000 cheering fans.
Dr Perry believes this will be "useful" when Kipchoge is at his "absolute limit".
Dr Perry points out that there is no hard evidence that cheering crowds can compel runners to better times.
She says that studies which have shown a supportive spectator benefit are based on the effect of home crowds at team games such as rugby or football, and this benefit is thought to be due to the unconscious impact this has on the referee.
But as a marathon runner herself, Dr Perry says supporters are vital.
"Crowd support helps you to focus" and "gives you the extra edge," she says.
And when every second counts, the "extra edge" is what Kipchoge needs.
Dr Perry adds studies have shown that when athletes are exposed to smiling faces, their perception of effort drops, again, potentially providing Kipchoge with the extra edge he needs.
Every little thing counts
When seconds will make or break months of painstaking training, every little thing counts, right down (literally) to footwear.
A marathon is 26.2 miles, roughly translating to a lot of strides taken.
Kipchoge takes an average of around 185 strides per minute when running marathons, so should he run 1 hour and 59 minutes, that would be more than 22,000 steps.
The best footwear is essential.
Back in May 2017, Nike developed its Vaporfly 4% shoes which all the Breaking 2 competitors wore.
The shoes - which have become popular with distance runners - are engineered to make an athlete 4% faster.
The shoes contain - amongst other changes - a carbon fibre plate which helps to propel the wearer forwards.
This time Nike have developed the ZoomX Vaporfly Next% which claim to make an athlete an extra 1% faster.
Kipchoge needs to run 0.36% faster than his Monza time to break two hours.
It'll be something to smile about if Kipchoge does manage it
Kipchoge is renowned for running with a smile on his face, often when his competitors are grimacing and even he must be in a fair bit of pain.
Following the Monza attempt, in a race that had pushed Kipchoge to his absolute limits, he later said he was smiling to relax and work through the pain, employing a strategy some runners have long believed to be true: that smiling while running can help you to run more efficiently.
Should he achieve the challenge, Kipchoge will be beaming as he crosses the finish line.
For the world's fastest marathon runner, it's not about setting a new World Record (he already has one of those) but about providing inspiration for others.
"I want to show the world that when you trust in something and have faith in what you are doing you will automatically achieve it, whether you’re a runner a teacher or a lawyer," he said ahead of the event.
"It will mean a lot when I run under two hours.
"My main message to all the people in the world is: no human is limited."