Running is the most elemental sport. The equipment is simple: shoes, socks, shorts, shirt. The activity is natural. We once ran after antelopes on the savannah, and we now run around playgrounds as kids. For the most part, we compete against ourselves. And because it’s so personal, and so elemental, the inevitable decline that comes with age can be wrenching.

Aging reduces our performance at everything athletic, but sometimes it’s hard to make out what’s happening. The ball doesn’t seem to go quite as far; the racket or the bat doesn’t swing quite as fast. The back starts to ache a bit. But the more complex a sport is, the more confounding factors—or excuses—there are.

In running, the evidence is right there: We ran the same distance and we went slower. What took 10 minutes now takes 11; what took three hours now takes three and a half. The evidence of the damage that time does to our bodies shows up implacably on our watches. I’ve been running for decades, and after almost every slow race I worry that I’ve stepped onto an escalator headed inexorably downward.

About the Author Nicholas Thompson (@nxthompson) is WIRED's editor in chief, cofounder of The Atavist Magazine, and author of The Hawk and the Dove: Paul Nitze, George Kennan, and the History of the Cold War.

There isn’t, of course, a magic age at which runners start to get worse. Most of the men’s and women’s distance-running records were set by people in their mid-twenties. But those are the elites; they use their bodies hard, and their speed, when it goes, tends to go quickly.

Recreational runners, on the other hand, may slow more slowly. Science suggests that the best age to run may be 27 for men and 29 for women. The men’s world record in the marathon was just set by Eliud Kipchoge, age 33. A study of marathoners in Stockholm pegged the decline to age 34. Whatever the exact age, by the time you’re eligible to run for president, you’ve likely started to fade.

I’m quick but nowhere near elite. I ran well in high school and then badly in college. I bailed at 18 and didn’t really start again for a decade. I ran a 2:43 marathon at age 30, a moment when I was old enough to start declining but inexperienced enough to keep improving. For the next nine years these two forces—experience propelling me forward and age pushing me backward—stayed in balance. I ran marathon after marathon in roughly the same time, sometimes a bit faster, sometimes a bit slower.

But when I turned 39, I finally started to slow: Each marathon got worse. Last year, at age 42, I did a little better than the year before, but the trend line did not look good.

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This year, I set a slightly fuzzy goal of two hours plus my age in minutes, a barrier I had never quite broken. I would turn 43 in the summer, which meant I wanted to run a fall marathon in 2:43. As I was beginning to plan, late in the spring, I got an unexpected note from Nike saying that they often connect non-elite runners to elite coaches. Did I want to run the Chicago Marathon under their tutelage in October? Of course I did. And that’s how I set out on a quest to understand the deeper science of running, aging, and performance.

I got on a conference call with three experts—Brett Kirby, a sports scientist with the calm demeanor and seeming wisdom of Obi-Wan; Stephen Finley, a big-hearted coach who nearly made the Olympic trials in the steeplechase; and Joe Holder, a physical trainer and former University of Pennsylvania wide receiver whose other clients include Naomi Campbell.

I told them how I’d worked out in the past and what my goals were. I wanted to get faster, but I didn’t want to spend any more time on the sport. I sent them a link to my old online logs and told them that I now tracked every run in Strava. They assured me that there was a way to beat the ravages of the years with science and a little bit of math.

The physiology of running can be broken down into three parts. There’s the body’s fitness: how fast you can get oxygen to the muscles and how fast you can go before lactate accumulates in the blood. Then there’s running economy: the efficiency with which you move. And then there’s mass: how much you weigh. Multiply fitness by running economy and divide by mass. That’s how fast you’ll go.