For a beginner runner to get into running because in 6-12 months they want to run an ultra-endurance race (or even a marathon) is—to put it mildly—folly.

There’s a reason the marathon is the final event in the Olympics: It’s by far the hardest. A recent The New Yorker article reports on one athlete trying to describe the experience of running a 2:10 marathon: “You feel like you will die. No, actually die.”

There are fundamental differences between the endurance sports and the power sports. Oftentimes, when discussing these differences, people think about what gives an athlete a competitive edge: for power sports, it’s higher concentrations of Type I muscle fibers. For endurance sports, it’s more mitochondria, and a greater oxygen carrying capacity.

This is important, but it’s not what I’m talking about. I’m talking about understanding the endurance sports by attempting to discuss what “endurance” is—not human endurance, or endurance at sports, but rather what “endurance” means in a fundamental sense. And for that, I find it best to discuss extremes.

Take a power sport: the 100 meter sprints, for example. Usain Bolt is a phenomenal athlete. There’s no question about it. And there’s no question that there’s a certain glory to be had in being the fastest human being on the planet—glory that is simply not available to the marathoner. Let’s set that aside. What would have to happen for Bolt to be unable to continue competing?  In other words, what would “catastrophic system failure” mean for Bolt?

My answer is: an ACL injury, or a torn hamstring, probably. In other words, something breaks.

Now let’s look at the marathon. Rarely does something break in that way in the endurance sports. There’s plenty of microdamage—achilles tendinitis, stress fractures, chronic fatigue, etc. But when something breaks, truly breaks to a point where the person cannot compete (in the “catastrophic system failure” sense discussed above), what does that look like? It’s typically the entire system that fails. Take a gander at a list (compiled off the top of my head) of the quintessential ailments you see in a marathon:

• Extreme dehydration
• EAH/EAHE (Exercise Associated Hyponatremia/Hyponatremic Encephalopathy)
• Heart attack
• Kidney failure
• Heatstroke
• Respiratory infections

What these issues all have in common is that they’re systemic failures—they’re what happens when the body as a whole, rather than a specific part (say, the hamstring) can’t cope with the event. In other words, they’re what you get when the body starts to come apart at the seams.

The best way to think of this difference is that when you bust a hamstring (or even break your spine in certain places) you can still use your body as a whole except for the part you broke. But when you get any of the illnesses that typically occur during a marathon, it’s the entire body that is put out of commission—sometimes permanently.

To put it simply, we can think of speed and power as a question of how powerful the body is. And while speed and power have tons of importance in the endurance sports, we can say that endurance is primarily a question of how good the body is at holding itself together. In other words, endurance is a test of the body’s fundamental integrity: of how much stress can you subject it to for how long without any substantial collapses in any critical processes.

And this is the main difference between the endurance and the power sports. In the power sports, the body has to be very, well, powerful, but it doesn’t have to be all that good at holding itself together—at least not in ways that relate to the ailments described above. After all, the power sports only ask the body to perform for a few moments: it stops before it becomes dehydrated, or before enough lactate builds up that the kidneys fail, or before the lungs become stressed enough that they become susceptible to infection (etcetera, etcetera).

But that’s not the case in the endurance sports. The body is going to be in activity for a very long time. If any of its systems (respiratory system, cardiovascular system, etc.) are working at different rates, some of those systems are going to get tired first. This is a problem: those systems were only active in the first place is because they were providing a critical service to the body’s endurance performance.

When one of those systems fails, some critical process associated with it also stops. If the body continues activity in this state, critical processes start falling like dominoes. And the body starts coming apart at the seams.

It’s not that endurance sport are “better” or “more of a sport” than power sports. But it is the case that being highly successful at an endurance sport takes much more time, much more consistency, and much more athletic maturity than to be highly successful at a power sport. This is why, for example, it is not uncommon to see 19 and 20 year old athletes competing in power sports at the Olympic level—the 400m, the 1500m, etc.

It’s usually those very same athletes who, 10 or 15 years later, are running marathons. Once their athletic career was already taking off, it took their body an additional 10 to 15 years to be physiologically organized and cohesive enough to run a marathon.

On the other hand, any athlete who is seriously contending for a medal at an endurance sport at 20 years of age, is a unicorn. Either they’re already so athletically mature that they’ll have a wildly successful career ahead of them, or they have already pushed themselves so far, so fast, that decades of chronic illness and overtraining are already on the horizon.