Out there, pounding the hard yards, losing weight, getting fit, ready for the run, joggers owe it all to one man who died a quarter of a century ago, Jim Fixx.
Running would have Sartre fuming in his grave, and put the sport's first prophet in one.
Twenty five years ago today, a formerly fat magazine editor named Jim Fixx was found dead by the side of a road in the New England countryside.
Fixx was a self-made phenomenon, a nobody who somehow became world-famous for losing a little weight, giving up cigarettes, and jogging down the street for an hour a day.
He put his hairy legs on the cover of a rather mediocre book and declared that running was the secret for health, wealth and an active sex life. In his case, it was true.
He made a fortune from one of the bestselling hardback books of all time, but suffered a massive heart attack seven years later in the middle of a run.
That should have been the end of that. Any scientist who held up a vial, proclaimed "voila, the formula for eternal life" and then keeled over after downing the contents would guarantee that no one would ever touch the stuff again.
Fixx's own book, in hindsight, is almost pulsing with public-safety warnings. The cover of "The Complete Book of Running" is the flaming red of an emergency evacuation signal.
The foreword is titled "On the Subversive Nature of This Book", hinting at trouble ahead like the demolished cars that some US police forces put by the side of the road to warn drivers what happens if you get a little too seduced by speed.
But instead of slowing down, people by the millions have been galloping on. Far from subversive, the marathon is now downright suburban.
It has passed the celebrity smell test, with world-class eaters and partiers like Gordon Ramsey, Oprah, and Puff Daddy finishing marathons.
And it's gone on to become a stamp of political earnestness. In the US, every presidential campaign since golfer Richard Nixon was run out of office has had a serious set of legs on the ticket.
Jimmy Carter, Michael Dukakis, the Bushes, Al Gore, John Edwards... even Bill Clinton strapped on the trainers and at least tried to look the part.
Barack Obama was "accidentally" photographed running in Hawaii just before taking office, and Sarah Palin still has time for her hobby. The same week she quit as governor, she appeared wrapped in the American flag in Runner's World magazine. Mikula Dzurinda, prime minister of Slovakia, managed to steal time from running the country to run the Malokarpatsky Marathon.
The final straw was Nicolas Sarkozy's scandalous sweat streaks. The French president could marry a model who had posed nude with zero political fallout. But huff around Paris in shorts?
"A right wing conspiracy," grumbled Liberation, Sartre's own news organ. All this jogging, Sarkozy's liberal critics say, is an attempt to push a conservative agenda by broadcasting the subliminal command to quit messing about with art and literature and empathy and get down to survival-of-the-fittest business.
It's almost a clear declaration of image war with those iconic photos of Sartre himself - legs crossed, sipping a coffee beneath clouds of Gauloises smoke, outliving Jim Fixx by a languid quarter-century.
The conspiracy theorists are right, of course. At least about one thing - the running boom is certainly a sign that trouble is afoot.
Three times in modern history, distance-running has skyrocketed. Each time, it's been in the midst of a major crisis.
In the US, the first boom came during the Great Depression. The next was in the 1970s, during a recession, race riots, assassinations, and the war in Vietnam.
And the third boom? One year after the 9/11 attacks, trail running suddenly became the fastest-growing outdoor sport in the US.
Maybe it's a coincidence. Or maybe there's a trigger in the human psyche, a pre-coded response that activates our first and greatest survival skill when we sense the raptors approaching.
In terms of stress relief and sensual pleasure, running is what you have before you're old enough for sex. The equipment and desire come factory-installed - all you have to do is let her rip and hang on for the ride.
And is it also a coincidence that some of the most enlightened people on the planet earth are the greatest ultra-distance runners?
Not just Nelson Mandela (a cross-country runner who still jogged seven miles a day in place in his cell), or Abraham Lincoln (known for "beating all the other boys in a foot race").
The Tarahumara Indians, a lost tribe who live at the bottom of Mexico's wild Copper Canyons, routinely run races of 200 miles or more, without shoes and on the balls of their feet. They are celebrated for their lack of obesity, diabetes and depression, and their amazing longevity. They also avoid the runner's curse of knee and foot injuries.
But if running is the magic pill, what happened to Jim Fixx? As it turned out, for years Fixx had smoked two packs a day and wined and dined heartily. He'd bloated up from 150 to 215 pounds before he finally started running at 35, the same age his father was when he suffered his first heart attack.
Despite his dad's death at 43, Fixx wouldn't do anything about his super-high cholesterol or even get a treadmill stress test, as a cardiologist friend begged. The autopsy would later reveal that three of Fixx's cardiac arteries were almost completely blocked.
Running used to be a necessity, the only way early humans could survive and thrive and spread across the planet. You ran to eat and to avoid being eaten. You ran to find a mate and impress her, and with her you ran off to start a new life together. You had to love running, or you wouldn't live to love anything else.
Running used to be vital for survival - and now, as tens of thousands turn out for the London Marathon and millions more fill the parks after work, we're instinctively realising that it still is.
Christopher McDougall is author of Born to Run: The rise of ultra-running and the super-athlete tribe. After years of injuries, he converted to running barefoot.
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