I started running only a few weeks after my 12th birthday. My best friend and I had impulsively decided to join the middle school cross-country team to get out of gym class.

I didn't even know what cross-country was at the time — before the first day of practice I suspected it would involve some form of skiing equipment. Being the two youngest and slowest runners on the team, my friend and I started training after school by running with rocks in our backpacks. As far as I knew, we were merely holding out until the other was ready to call it quits.

I do not know how those painful and dangerous mid-October afternoon training sessions somehow became the start of my longest, most dedicated relationship with any activity. But 14 years later, I still get single-word texts from the former teammates I trained with on that grassy, mile-and-a-half-long pitch in the Bronx: Run? That's all I need to read and I'm already reaching for my shoes.

The first question people ask after learning that I run is whether I am training for a marathon. I'm not. Nor am I training for a half-marathon, although on occasion my former teammates and I do go out and run 13 miles. I am not running to stay in shape, but I do wonder what might happen if I decided to stop.

For many, the act of running is a way to stay thin, or to fight off heart disease or stay active in a strange attempt at getting over a breakup, or even to catch a train whose doors are about to close. But I don't run to be alone and escape into my headphones, or to zone out and meditate on my troubles. I am not an introvert who uses exercise as a break from human contact.

For me, running is itself the destination. It has become my way of balancing the scales, and at some point since 2002 I started to feel most at home when my body was most worn out. I count my life in miles.

I run because after all these years I have learned to find a measure of serenity in the act of running. Somewhere in the past decade I started to find pleasure in this singularly painful activity.

In the past few years, long-distance running has gone from a trademark of masochists and compulsive dieters to a national pastime for people in their 20s and 30s. Runner's World estimates that the number of people who complete half-marathon races has increased almost tenfold, from 300,000 to more than 2 million since 1990 when I was born.

According to Running USA's state of the sport report, the number of marathon finishers has seen a growth of nearly 50 percent in the past 10 years. Distance running has even become something of a casual hobby, with a slew of apps and online resources dedicated to getting people in marathon shape in only a few months.

While new runners may be familiar with the pleasure of leaving their house on the first day of spring with nothing but shoes and shorts and a key, they cannot possibly be aware of the ways in which running will change their lives. When practiced over great periods of time — not simply over many miles — I have found that something about the sport changes you, perhaps even the way some experts believe that growing up bilingual alters brain function.

I am only now beginning to understand the long-term effects that running has had on my life. Running long distance means that some ligament or muscle of the body will always be sore, even when not running, and so runners learn to interpret all manner of pain in only two volumes: chronic or fleeting. Anything less than chronic can be pushed through, fought with, massaged, foam-rolled out of the body, or very simply ignored. This is because to become a better runner, one must repeatedly push oneself beyond one's threshold. Can we really be surprised that this instinct spills into other corners of our lives?

This past fall I waited three days after a rock collapsed out from under me on a hike before going to see a doctor, only to learn that I had broken two ribs. Last year a friend and longtime distance runner waited so long to see a dentist about tooth pain that the nerve in her tooth died. Dr. Jordan Metzl, a sports medicine specialist and author of Running Strong, believes that runners condition themselves to ignore pain.

"There are some things about running that change physiology and psychology," he says. "Runners can tough stuff out. Their pain response is a little dulled over time. They can suppress things, push out longer, because they're used to it."

For years I assumed that some of my most pronounced quirks were personal idiosyncrasies — leaving the house without a coat in single-degree temperatures, forgoing the use of oven mitts unless a pan is hot enough to burn skin, trudging almost unaware through bouts of fever or bronchitis — only to realize that these very same behaviors were practiced by many of the long-distance runners I know. It is not that we cannot feel pain, but we simply choose not to care.

Those new runners who work best during short bursts should switch to CrossFit before the changes become irreversible Yet at the same time, a stiff hamstring can derail almost anything. These sorts of workout-interrupting discomforts are treated with the same urgency as grave illnesses, and must be addressed immediately. Runners can ignore almost any pains save those that — however faint or indistinct — presage a break or a tear.

It is fitting that this acquired pain threshold is accompanied by a greater psychological endurance as well. Runners will frequently train in negative splits — a term used to describe the practice of running each mile at a faster pace than the last. This sort of repeated aggression against one's physical limits can only happen when one has the endurance for it.

This means that because runners are able to sustain an activity over the course of 9 miles, they also find themselves particularly well-adapted to working intensely for long stretches of time.

"Psychologically, it's the same kind of thing as changing your pain response," Metzl says. "It's socialization. You train yourself to develop this form of concentration, which is necessary for long-distance running."

For years I avoided learning to drive. Yet after finally getting my license at 25, I was surprised to find myself well-suited to very long bouts behind the wheel, able to drive for many hours before beginning to fade.

But this socialization, as Metzl calls it, can come with obvious drawbacks. Shorter tasks can begin to feel tedious and uncomfortable. On longer runs you will often feel stiff for the first mile, only to suddenly realize that somewhere in the middle of the second your stride has become open and elastic.

Running teaches you to look for your stride in the second mile. That's where you'll find it. Nothing before that moment counts. Mile-long sessions are exercises in frustration. My mind is always aimed toward a long haul. Those new runners who work best during short bursts should switch to CrossFit before the changes become irreversible.

My body is quite unlike the one I would have had if my friend and I had decided to quit the team halfway through October 2002. More than that: It is unlike the one my genes would have predicted. I have yet to join the men in my family, who, even when physically fit, seem to get a little rounder in their 20s. I do not know what my resting natural body is like. It and I are strangers.

My doctor recently told me that we no longer know what my genetic cholesterol predisposition looks like because I have been running for so long. Physicians have long thought I was naturally hypoglycemic, but a new doctor suggested that I may very simply have been running so much that my body's blood sugar is kept artificially low. I could even be prediabetic, he joked, but we might never know, because I'm essentially always starving.

Those of us who have been running since before puberty are distinctly out of touch with our baselines in every physical sense. We do not know what normal is. It is like a system whose equilibrium has been attacked. Even those of us who run for miles in the woods nevertheless remain detached from the natural order of our bodies.

What a strange, lonely idea, that running should pull us further away from our true natural selves, the selves we are at rest — the selves we were meant, in every sense, to be. Our body's universe has been completely disturbed. Yes, my body is a lot thinner than it might have otherwise been at 25, but it is also pockmarked with injuries that I never would have had.

Running, one orthopedist insisted, is highly injurious on the body. I am incredibly injury prone — a torn Achilles tendon in 2014, broken ribs this fall, chronic IT band problems, quad weakness, years of knee pain, neck stiffness.

My first injury was the strange growth of bone and ligament by my knee called Osgood-Schlatter, a common ailment in teenagers that took me out of commission for the last season of middle school track. My second serious injury was freshman year of college, which turned out to be another common runner's issue called patellofemoral pain syndrome.

Running long term has trashed certain parts of my body. Runners subconsciously learn to downplay the severity of symptoms when being checked out for an injury, hoping somehow that underreporting pain will force the universe to accept your hopeful words as gospel, that speaking this incantation of backward superstition will somehow make it true and that you will get the green light from your doctor and go back to running sooner than expected simply because you wished it.

It won't. But you'll go out running anyway, and you might even hurt yourself.

Even Metzl, whose research suggests that runners have lower rates of depression and lower risk of heart disease and cancer, understands the risks associated with running. Distance training can cause loss of fast-twitch muscles, and so running, he says, should be accompanied by strength training to aid in injury prevention.

The ever-present risk of injury stimulates hyperawareness of each joint. The creak in your left knee is like the voice of an old friend; you can never entirely forget the specter of a dormant inflammation in your hip, kept at bay only with meticulous stretching and icing. You silently groan before a long flight of stairs.

But with this also comes a heightened acuity for understanding the other mechanics of your body. Long-term runners do not need GPS watches to time their mile paces or know how far they've run, or heart rate monitors. Calculating these metrics without the use of technology is a learned instinct and, when in the company of new runners, something like a party trick that amazes them as though you had just pulled a quarter out from behind their ear.

After all these years, it is unclear to me whether my body is a finely tuned microchip or a lug nut that has been stripped of its edges from years of abuse. Whatever the case, my body yearns for those moments when it feels most worn out after a long run. Because my notion of pain has been changed over time, so too has my notion of pleasure. From this source of pain the body procures absolute pleasure.

With time you begin to revel in these moments when the movement of each joint is sluggish as though injected with grains of wet sand and glassy mineral, and when a flush of warm blood sweeps across your face like a mask. The air draws violently through your nostrils, carrying with it a noxious vapor of salty moisture lifting up from your skin, mixed with dirt kicked up from your shoes.

In the next hour you will hardly be able to go from one corner of your apartment to the next. Your body has been ground down to a powder, rubbed raw like an element stripped of its valence electrons.

For a few moments, as you pick yourself up from the slumped position and fight through the urge to vomit and catch your breath, you live completely unencumbered This very sensation was one I used to dread during my first years as a runner. It is perhaps this same fear that keeps so many people from taking up running to begin with. And yet I have come, with time, to feel most like myself during these moments. The world changes entirely when you are worn out from a long run, and you almost begin to feel new again, as though during each negative split mile you were shedding not only entire seconds from your pace, but parts of yourself as well. Those parts are forever lost to the run.

For a few moments, as you pick yourself up from the slumped position and fight through the urge to vomit and catch your breath, you live completely unencumbered. The post-run daze, which for new runners is understood only as the moment of greatest discomfort, is the closest thing to serenity that exists for old runners. It is chased, literally, for miles.

Time teaches you to find a home in this moment — a sense of belonging, or even a sense of natural order — as though it is only then that the real fog clears.

Stranger yet, in this clearness you find changes in yourself. You do not feel the need to brush off the thin film of dirt against your palms. In winter you are warm, and in summer cool. The thirst you felt in the third mile has vanished. Aches from the fourth mile have died down. You feel only the buzz, as though nothing but the body exists, and you are drawn to basic indulgences that would have been an assault to your whole being after a workout in your early years as a runner. Beer, drunk in this state, tastes like the sweetest nectar. You drink it as though nothing else matters, and wonder, as though it were the first you ever had: Should anything on Earth ever be allowed to feel this good?

Alexander Aciman is a writer living in New York. His work has appeared in the Wall Street Journal, the New York Times, Tablet, the New Republic, the Paris Review, and elsewhere. Follow him on Twitter.

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