For most runners, finishing a marathon - regardless of the time it took - is a major achievement in itself.

This weekend, more than 59,000 people ran the London Marathon, including Wicked star Cynthia Erivo and former England footballer Tony Adams.

But before the club runners and those chasing personal bests were unleashed onto the capital's roads, a small group of elite runners headed off - and when two of them crossed the finish line less than two hours later, the world was left in disbelief. 

Yesterday's Marathon winner Sabastian Sawe stunned spectators, not only with his pace but also his endurance. 

Unlike most elite marathoners, whose form gets worse as the race progresses, Sawe maintained a stable stride even in the final miles.

In simple terms, his body appeared to resist the usual signs of fatigue that slow other runners.

Sawe, 31, from Kenya, stopped the clock at 1:59:30, with Yomif Kejelcha just 11 seconds behind in 1:59:41.

Until now, the closest official time was Kelvin Kiptum's 2:00:35 set in 2023.

Even the legendary Eliud Kipchoge, who famously broke two hours in 2019, did so in a controlled exhibition rather than a competitive race.

Sawe averaged around 4:33 per mile over 26.2 miles, holding speeds above 13mph from start to finish.

'He has no fear. He knows his strength,' said his uncle, former athlete Abraham Chepkirwok.

'He doesn't panic in races. Even when others are suffering, he stays calm.'

Sawe himself was typically understated afterwards. 'I am feeling good, I am so happy,' he said. 'It is a day to remember. I have shown that nothing is not possible.'

Sports scientists say the explanation begins with three core physiological factors.

Dr Richard Blagrove, an expert in endurance physiology at Loughborough University, explains that elite marathon performance is usually determined by maximal oxygen uptake - known as VO2max - the ability to sustain that oxygen use, and running efficiency.

Based on laboratory work with elite athletes, Blagrove believes Sawe sits at the extreme end of all three.

'He would likely have a VO2max in the high 70s to low 80s, be able to sustain more than 90 per cent of that for around two hours, and have exceptional running economy,' he explained.

But Blagrove says there is now a fourth factor becoming just as important: durability.

'In most athletes, efficiency starts to drop after around an hour of hard running,' he said.

'But in rare cases, that deterioration is much smaller. The athlete at the start line is almost the same athlete you see at the finish.'

That, he suggests, may be what separates Sawe from the rest.

Behind that physiological profile sits one of the most demanding training structures in elite sport.

According to Runner's World, Sawe typically runs 125 to 150 miles per week in a marathon buildup.

Most of it is easy mileage, but it is layered with two high-intensity sessions and a weekly long run of around 40km, often performed closer to race effort than traditional endurance training.

That combination of volume and intensity is designed to push his physiological ceiling while forcing the body to recover faster under repeated stress.

'He looks unusually relaxed even during hard efforts,' one insider said. 'Even near-race pace sessions can look controlled.'

Sports scientist Dr Ross Tucker says that the level of mileage is not new in elite running but the way it is now tolerated may be changing.

'Very high mileage has always been part of elite marathon training,' he said.

'What we don't fully understand is how athletes are now able to handle it in combination with higher intensity work.'

Tucker suggests advances in footwear may be part of the explanation.

'Carbon-plated shoes with improved cushioning may allow athletes to absorb more training load without the same injury or fatigue cost as before,' he said.

Like many Kenyan runners, Sawe also trains at altitude in the Rift Valley.

Lower oxygen levels force the body to produce more red blood cells, improving oxygen delivery when racing at sea level.

Over time, this can reshape endurance capacity and efficiency under stress.

Tucker adds that long-term altitude background may also play a role.

'It's not just training camps,' he said. 'There is evidence that altitude ancestry over generations may contribute to how athletes tolerate extreme endurance loads.'

Put together, experts say Sawe represents a rare combination of physiology, training tolerance and efficiency.

Blagrove puts it simply: a runner whose body not only performs at an extreme level but appears to maintain that level for longer than most others can sustain it.

That, more than anything, is what has left the sport questioning how far the limits of marathon running can now be pushed.