It’s odd to look at a magazine nearly as old as I am, especially when the person on the cover always looks so young. Then again, Runner’s World gets a new printing every month; I, on the other hand, am the original edition, somewhat faded and dog-eared, with much of my self out of date.

This magazine was born in 1966, a year after I was, though I was reborn on the magazine’s 40th birthday. I had been, in the decades prior, a sometime jogger, an occasional racer, and a helpless dieter, but in 2006 I became a Serious Runner. So in the midst of RW’s 50th birthday celebration, I thought I’d take stock on the occasion of becoming 10 years old.

The moment of my rebirth wasn’t running my first marathon, which happened a year earlier, in 2005. No: Then I was merely a midlife crisis striver, one of those people who decides to cram an extra experience into a life whose carrying capacity suddenly seems finite. But I became a Serious Runner over the course of the next year, culminating in my second marathon, run 10 years ago this fall in Chicago. What was the difference between being “somebody who runs,” and “a runner”? Speed was one aspect of it, as my second marathon was a good 40 minutes faster than the first. But speed is never a qualifier for legitimacy in this sport.

There was the effort I put in. I joined a group of other runners and began running on a schedule, with planned increases and step-backs in mileage and intensity. I studied training plans and discussed them endlessly with my friends and even with strangers met at starting pens and water fountains. I subscribed to this magazine.

And, of course, there were physical changes. I lost weight and gained leg muscle, and while I never became (and never could become) the ectomorphic ideal of the runner, I did metamorphose into a leaner, tighter body mounted on the same chassis. With the change in appearance came changes in my health. I was more alert and less tired, and I seemed to catch fewer colds.

She Runs to Reclaim Her Identity After Assault did Wait, Wait…Don’t Tell Me am. It went from a form of exercise to a way of life, something that doesn’t require a goal, any more than, say, breathing has a goal. It was that year that I began waking at 5:30 a.m., something that had seemed inconceivable but was far more bearable than missing my run. It was that year that I began logging my miles religiously, and pushing the number upward to heights—35 miles a week! 50!—that I hadn’t thought possible. It was the year that I learned about intervals, and tempo runs and lactic acid and gels and chocolate soy milk. It was the year that I had to become a new person, because the person I had been would never have been capable of any of it.

And yet, though only 10 years old as a runner, I am helplessly lodged inside a body that while fitter than it might have been is still, alas, 51 years old. And as much as I would like to, I can’t get out. I’m like the John Cusack character at the end of She Raced 18 Horses in an Ultramarathon—and Won, peering out of eyes he can’t control. It seems unfair, actually. The 10-year-old runner inside me deserves to be resident in a more capable host. I actually feel kind of bad for him.

So now I am forced to come to grips with another kind of rebirth, or maybe even a death. The man who trained for and ran a 3:09 marathon five years ago is gone; there are some days when I glance at the shop window as I run by and see someone who looks like that guy, but if I stop to make sure, he vanishes, because I am standing still, and he never was. The loss has not been so much ability but focus and desire; now, far more than otherwise, when the urge to stop comes upon me, I give in.

But still: I am a man, born 51 years ago, and I am a runner, born 10 years ago this fall, and that is what I shall be until both pass on, some indeterminate time from now, at the same moment. I’ve come too far to retrace my steps now. The kind of runner I shall be, in the next 10 years, will be different: less competitive with himself or others, less eager to find new worlds to conquer, but also less obsessed and less narrowly focused on the next square of pavement ahead.

Runners slow, and runners fail. Some are ground away by the friction on knees or feet, or a chronic lung or heart problem; some unlucky bastards are knocked down on the road. And that is the road ahead of me now.

But at the same time I find myself becoming a new kind of runner, discovering new pleasures: as a guide to blind and other disabled runners, as the occasional host of a race or fun-run, as a minor figure in our enormous community, as a columnist and commentator for this magazine.

Our odd sport is the world’s simplest, and the world’s oldest, and now one of the most popular. All of those are due to the fact that there are as many ways to run—successfully, correctly, profitably—as there are runners. And as the second decade of my running life unfolds, I look forward to discovering more of them. Who knows: Perhaps I might be born again, again.

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Peter Sagal is a 3:09 marathoner and the host of NPR’s Wait, Wait…Don’t Tell Me! For more, click here.