She Ran a Sub-3 Marathon at 25. Then She Died 497 Meters From a Half-Marathon Finish.

At 8:49 a.m. on October 11, 2026, in Seoul, a runner in the 2026 Seoul Race half marathon collapsed at the 20.6-kilometer mark. She was 497 meters from the finish line.

Her name was Ryu Ji-won. She was 25 years old, a graduate student at KAIST, and her research focus was AI semiconductors. In Korean running circles, she was a minor running influencer.

Medical staff on site immediately began CPR. An ambulance took her to the hospital. She never woke up.

Seven months earlier, in March 2026, she had completed the Seoul Marathon in 2:56:32, breaking the three-hour barrier and achieving a “sub-3” finish. From a sub-3 full marathon to collapsing near the end of a half marathon, less than seven months passed.

The exact cause of death has not been released. But based on what is publicly known about her routine and the medical research on endurance running, three factors likely combined: extreme fatigue, sleep deprivation, and the final sprint of a race.

“Your life seems to be only research and running”

Ryu Ji-won was born in 2001. She completed her undergraduate degree at Ewha Womans University and then entered KAIST for her master’s. In March 2026, she appeared on a running YouTube channel. In the opening, the host called her “a comet that suddenly appeared in the marathon world.”

Her schedule was public. At 9 a.m. she entered the lab, attended classes, did research, and stayed busy all day. At 8 p.m. she went out to run for at least two hours. If her research tasks were not finished, she returned to the lab after training to keep working. She deliberately chose a dormitory next to the track, because “the moment I open my eyes, I can go out and run.”

When the host remarked, “Your life seems to be only research and running,” she laughed and said, “I guess so.”

At the time, that answer sounded like pride.

It is not an isolated case. The Korean Sleep Research Society’s 2024 report on sleep habits found that Koreans sleep an average of 6 hours 58 minutes, about 18% less than the OECD average of 8 hours 27 minutes. In Seoul, some young people say, “Between studying and part-time work, I only get three or four hours a night.”

Ryu Ji-won’s sleep time was likely even lower than that already-low average. High-intensity AI semiconductor research, two hours of running every night, and then returning to the lab to catch up on work. In a schedule like that, seven hours of sleep is almost a luxury.

Three Risks Stacked Together

Three factors likely combined.

First, the chronic erosion of the heart caused by long-term sleep deprivation.

Sleep deprivation damages the heart over time. A prospective study based on more than 410,000 participants in the UK Biobank found a clear link between sleeping less than seven hours and the risk of sudden cardiac arrest. The study also built an overall sleep score and found that for every one-point increase, the risk of cardiac arrest fell by 5%.

A Harvard epidemiological study of 70,000 women further quantified the risk. Compared with people who slept eight hours a day, those who slept less than five hours had a 40% higher probability of developing heart disease.

Liu Wei, director of cardiovascular medicine at Beijing Jishuitan Hospital, described the mechanism: “Overfatigue, long-term sleep deprivation, and high mental tension cause sustained sympathetic nervous system excitation, speeding up heart rate, increasing myocardial oxygen consumption, and continuously aggravating underlying cardiovascular damage. In this state, high-intensity exercise brings a double fatal risk.” Sweating during exercise raises blood viscosity, and high-speed blood flow can cause soft plaques in the coronary arteries to rupture, rapidly forming a thrombus. High-intensity exercise under fatigue can also trigger severe vascular spasm, leading to acute occlusion. Both paths can ultimately induce malignant ventricular arrhythmia.

Second, the physiological critical point in the final stage of a marathon.

A multicenter study published in The New England Journal of Medicine, the RACER study, analyzed data from about 10.9 million marathon and half marathon participants between 2000 and 2010. The results showed that nearly 80% of cardiac arrest events occurred in the last 25% of the race or shortly after the finish.

When a person runs for more than two hours, with heart rate sustained at 160 beats per minute or higher, the heart is already operating near its limit. Heavy sweating causes the body to lose water and electrolytes such as sodium and potassium. Blood gradually becomes more viscous. At this point, the myocardium is on the edge of fatigue and oxygen deprivation, and the heart’s electrical activity becomes unstable because of electrolyte imbalance.

The final sprint before the finish line causes extreme sympathetic excitation and a sudden further rise in heart rate. In medical terms, this environment is highly likely to trigger fatal arrhythmia. Ventricular fibrillation occurs in an instant. The heart loses its ability to pump blood effectively.

The place where Ryu Ji-won collapsed, the 20.6-kilometer mark, 497 meters from the finish, was squarely in this most dangerous zone.

Third, an underlying, undetected heart problem.

Among young athletes under 35, the most common cause of sudden cardiac death is a congenital or hereditary cardiac abnormality. Hypertrophic cardiomyopathy is considered the most common structural cause of sudden cardiac death in young athletes. It is characterized by abnormal thickening of the heart muscle, which affects pumping function. Electrical conduction abnormalities that can cause arrhythmias, such as long QT syndrome and Brugada syndrome, are also important causes.

These abnormalities may be completely asymptomatic in daily life. Ordinary activities and short-distance running may never expose them. But once the body enters a state of sustained high load, the problem is rapidly amplified. Autopsy studies in recent years show that most young athletes who die from sudden cardiac death have no structural heart abnormality found at autopsy. This means that even if an echocardiogram is done, not all hidden risks can be detected.

The sudden death rate in marathons is extremely low, roughly 0.5 to 2 per 100,000 participants. But when it happens, the outcome is almost irreversible. For runners under 40, the survival rate after cardiac arrest is only 7%, less than one-seventh of the 47% survival rate for runners over 40.

How to Run Without Pushing the Heart Past Its Limit

The effect of marathon running on the heart depends on the dose. Moderate aerobic running is very beneficial for the cardiovascular system. But ultra-long endurance exercise does not increase benefits linearly. It may instead bring structural risks. A slow five-kilometer or ten-kilometer run strengthens the heart. Several consecutive hours of high-heart-rate running consumes the heart more than it strengthens it.

Many people treat the marathon as proof of discipline and willpower. They ignore that it is essentially an extreme stress test for the heart. The people most likely to be struck down are often those who consider themselves very healthy, run regularly, and have no obvious abnormalities on routine checkups. They may have mild myocardial hypertrophy, hidden coronary problems, or congenital electrical conduction abnormalities that are difficult to find in routine physical examinations.

Long-distance runners can assess their bodies in three ways.

Before the race, get the right tests, not just “some tests.”

A standard electrocardiogram and echocardiogram are the basic must-do items. An ECG is used to screen for arrhythmias and abnormal myocardial blood supply. An echocardiogram is used to evaluate cardiac structure. The two cannot replace each other. For runners over 40, those with a family history of heart disease, or those who usually have symptoms such as chest tightness or palpitations, it is recommended to add an exercise stress test or cardiopulmonary exercise test to evaluate how the heart responds under extreme conditions.

Routine screening has limits. Most young athletes who die from sudden cardiac death have no structural heart abnormality found at autopsy. Screening can rule out known risks, but it cannot 100% exclude all hidden dangers. Daily self-awareness and in-race judgment are equally important.

Before the race, if your body is not in good condition, do not run.

If you have stayed up too late, are overfatigued, sleep-deprived, have an unhealed cold or fever, or have consumed alcohol, you should not participate in a long-distance race. Running after staying up all night is especially dangerous. High-intensity exercise under fatigue is the strongest catalyst for sudden death.

During the race, recognize the signals and learn not to sprint at the finish.

The heart itself has no pain nerves. It will not warn you when it is being damaged. By the time it sends a signal in the most violent way, there is often no room left to turn back.

Warning signs exist. During exercise, if you experience chest tightness or chest pain, palpitations with dizziness or blurred vision, weakness in the hands and feet that does not obey commands, unexplained heavy sweating, or difficulty breathing, you should stop exercising immediately and seek help. These signals may appear minutes to hours before sudden death occurs.

Do not blindly sprint near the finish. The sudden rise in heart rate before a marathon finish is an important trigger for fatal arrhythmia. In the last half mile, choose a comfortable pace rather than an all-out sprint.

Running Itself Is Not Wrong

Ryu Ji-won once described her first serious running experience in an interview: “At that time, my heart really felt like it was about to explode, but my mood was especially refreshed. It felt different from before.”

She loved running. After the tragedy, Hwang Young-cho, the 1992 Barcelona Olympic men’s marathon gold medalist, spoke about it on his personal channel. He reminded long-distance runners to pay attention to the warning signs their bodies send during races, and not to force themselves past their physical limits in pursuit of a personal best.

Every runner should know this. Running should strengthen the body. It should not drain it. The heart does best with exercise that is moderate and sustainable over years. That may mean shorter runs rather than a 42-kilometer marathon or a constantly improving personal best.

Under Ryu Ji-won’s social media account, one runner left a message: “I hope you continue to run happily in heaven.”

Run safely.