100 Ks Later

You know marathons. Rome. Bla bla. In recent years, people in their droves have decided a 42KM run isn’t long enough. Enter: the ultramarathon.

Figures are hard to come by, but we know that between 1996 and 2018 participation rates in ultras increased by 1967%.

Source: RunRepeat 2020 Report. 

If our Insta feed’s anything to go by, that hockey stick’s a whole lot taller today. 

Any footrace over the distance of 42.2KM is considered an ultra. While there are records of soldiers and messengers covering long distances dating to the 1700s,  the proto-spirit of the ultra competition can be traced back to the London To Brighton (84KM) challenge – first raced by just two people in 1837 by John Townsend and Jack Berry.

The first proper Ultra Marathon was set up in 1921 by British war veteran Vic Clapham: The Comrades Marathon. Inspired by the London to Brighton races, having moved from England to South Africa aged 13, and subsequently fighting in World War 1, Vic created the race as a yearly memorial to South African soldiers killed in the war. The run is 87k, between Pietermaritzburg and Durban (they alternate each year for an “Up Run” or “Down Run”). It’s considered both the oldest and largest ultra–in 2025, 22,000 participants took it on. 

The whole story’s worth a read.

Built For This

You’ve likely heard the idea that humans summited the food chain due to our ability to run long distances –  “persistence hunting” – to wear out our prey. A dubious claim, championed by one specific anthropologist in 1984 and popularised in 2009 via the ultra-marathoner’s bible Born To Run.

An aside, but if you’re interested in learning more check these links out.

Whether or not we evolved to run mad distances, once you get beyond 50K, it’s no longer a viable survival strategy. The most extreme iteration is the Sri Chinmoy Self-Transcendence 3100 Mile Race. 10-15 people are selected each year to run 4989KM around a half-mile loop in New York, in 52 days. 

You can pick 5000km, or you can pick 2% of that, regardless, you are going to take an absolute battering with these distances. Let’s explore further, using 100KM as an example.

0–20KM: The Easy Lie

The first stretch is a confidence trick. Your body runs on stored glycogen. You’ve got about 90 minutes worth of fuel here. This part feels smooth, might even be fun. But your gut’s already changing as blood is routed to the legs, away from digestion. Eat too early, too heavy, and you’re setting a timebomb of nausea (at best) for later on. 

This is the reason why people say ultras are eating competitions disguised as footraces. Far more runners drop out due to stomach issues, or vomiting, than shredded quads or unbearable blisters.

20–60KM: Bonk O’Clock

This is where it starts to get serious. Glycogen drains, forcing your metabolism to use fat instead. This is what runners call bonking. Your fancy £200 runners turn into concrete slippers as your legs get heavy, fatigue sets in and you’re questioning your decisions. Breathing gets deeper, not because you’re crushing it but because fat costs more oxygen to process.

The best advice here is to slow down and try to take on carbs. Your gut’s already not functioning optimally as discussed earlier, so many opt to chug energy gels as they’re more easily digestible. This doesn’t shut off the fat oxidation, but it helps. This is the balancing act, you need the glycogen but you don’t want an upset stomach that nixes your months of training.

Calories are the cruel math of ultras: a 100K will torch between 6 and 8,000 cals. You can’t eat fast enough to replace them. Without discipline, the deficit caves you in: hypoglycaemia, dizziness, brain fog and the classic “ultra shuffle,” a technique to protect energy.

All the while, your immune system is slipping. NK cells are compromised due to the physical stress, temporarily decreasing their activity and increasing the window for infection shortly after the race. This is why in the preceding hours and days following a serious event, participants often get sick. It’s a tax you pay for the achievement. 

60–100KM: Into the Void

The teeth clench here–metaphorically and literally. You’ve been at it for hours. The body starts to raid its own muscle tissue for amino acids as it cannibalises itself to keep moving.

Calcium handling goes haywire, and cramping becomes more likely. Creatine kinase, a marker of muscle damage, spikes to astronomical levels. You’re basically bleeding from the inside out, cellularly speaking.

The central nervous system is really going through it now. Sleep deprivation, energy deficit, and sheer stress hijack perception. Hallucinations are common, one Badwater race saw ⅓ of runners see stuff that wasn’t there. The brain starts misfiring under load. The notion of “you’re only racing yourself,” never rings truer than now.

Kidneys stagger under the strain, too. With blood flow diverted elsewhere for hours, most ultrarunners show signs of temporary kidney injury. Eighty percent, by some counts. It heals, but the damage is real.

The Aftermath

Finishing doesn’t end the ordeal. The damage keeps rippling. Muscle breakdown markers up to 20x higher than marathoners. Immune suppression that lingers for days. Weight loss of 3-5% on average, depending how long you run (peaking at 11% for some). Total energy expenditure stays elevated. Your body’s still roaring at 5,000 kcal/day to repair, inflame, and rebuild. For a week you’ll be in an energy deficit, eating like a wolf just to claw back balance.

Yet here’s the paradox: despite the carnage, many ultrarunners finish elated. Mood spikes from dopamine and endocannabinoid rebound. It’s why people come back for more.

Finish Line

In spite of the physical onslaught that takes place during ultramarathons, people are flocking to events to try to push themselves to their very limits and test their mettle. It speaks to individuals looking to overclock the human operating system and reveal their true potential.

With the right preparation, these events can be life-affirming. But they need to be taken seriously if you’re keen to finish without… finishing yourself.