Like all the best inventions, it happened quite by accident. Paul Sinton-Hewitt, a keen amateur runner, was out one day when a dog stepped in front of his path as he was rounding a bend. He went flying and tore his hamstring, his dreams of competing in the London Marathon in dust. Single, recently sacked from his job at a software company, and now unable to run, he found himself as the loosest of loose ends. “My life,” as he would later put it, “was in the toilet.”

So one morning, Sinton-Hewitt persuaded a few of his running mates to participate in a timed run at Bushy Park in south-west London. Thirteen turned up. The results were calculated on paper, the finish tokens were washers from Halfords, and afterwards they all went to Caffe Nero in Teddington for a coffee and a chat. The month was October 2004, and thus was born the phenomenon that millions around the world would know as Parkrun.

Fourteen years on, the little time trial Sinton-Hewitt dreamed up on the banks of the Thames is quite simply the biggest mass-participation sporting event on Earth. On a good Saturday, around a quarter of a million runners will take part in 1,500 events spread over 20 countries. You may even be reading this on your way back from one, limbs weary, gait heavy, barcode sodden, and yet with the intensely satisfied sensation you only really get when you have conquered the true adversaries of the amateur athlete: the alarm clock, the hangover and the stubbornly adhesive appeal of Saturday Kitchen Live.

The runaway success of Parkrun has been well documented. So too its unique health and social benefits: in June, it was reported that GPs are being encouraged to prescribe Parkrun to overweight patients and those with chronic health conditions as an alternative to medication. But considerably less has been written about Parkrun from a sporting perspective, which on one level is perfectly understandable and on another curiously negligent. The thesis here is that Parkrun is not simply one of the biggest sporting events in the world, but one of the most important, largely because it entirely upends what we have long been told sport is about.

By way of example, take a look at your favourite sport homepage on an average Friday afternoon. There’s something about Paul Pogba talking. To its right, a story about a football manager talking. Just beneath it, a video of a boxer talking. Alongside, a story about an NFL player going to jail. Next to that, a story about sporting finance and politics. In the sidebar, innumerable quizzes and fun features bearing only the most tangential of relationships to people, in a place, doing athletic things. By and large, when we think of sport, we are thinking of something we watch and talk about: an industry, a product, a show curated for our consumption.

But there’s an alternative model of sport, one that may as well belong to an entirely different universe, and it’s exemplified by events like Parkrun. As elite sport bleeds ever more indistinguishably into the world of light entertainment, the idea of sport as something built from the bottom up, rather than sold from the top down, becomes ever more invaluable. As anyone who has ever done a Parkrun will know, events are run almost entirely by volunteers. The organisation itself employs just a couple of dozen staff. Unless you want a souvenir T-shirt, no money ever exchanges hands.

All of this is utterly anathema to the principles that govern sport and its gatekeepers in 2018. For decades, successive governments - in league with big business - have enjoyed great success in redefining sport according to the laws of the market. Watching live sport, whether on television or in person, has become increasingly unaffordable to poorer households. A decade of cuts to local authority funding has nibbled away at this country’s green spaces. The Olympic Games has been reimagined as a financial arms race, with billions of pounds in taxpayer money ploughed into Team GB’s sinister medal factory. In the space of a generation, sport has ceased to be something we do, and instead become something we pay for.

Running offers an alternative model of sport away from more commercialised forms ( Getty Images )

And so against this context, once a phenomenon like Parkrun develops enough momentum, it becomes possible to regard it not merely as a breath of fresh air, or a Nice Thing To Do At Weekends, but a genuinely disruptive force. Already, the first twinges of a backlash have been brewing: a number of local councils have voiced their disquiet at allowing Parkrun to use their parks for free, while two years ago a Parkrun in Gloucestershire was forced to shut down after the council demanded a staging fee. In the coming years, if the present rate of expansion continues, we will discover the true consequences of giving something away for free in a society where everything has its price.

In the meantime, the success of Parkrun continues unabated. Sinton-Hewitt’s latest wheeze, announced this week, is an attempt to reinvent the sportswear market, launching a range of athletic apparel in gender-neutral colours, free of sweatshop manufacturers, and marketed at all body shapes. Other sports are increasingly looking to Parkrun as an example of how they can drive their own mass-participation movements.

Governing bodies like the IAAF are beginning to recognise that for from being an uncontrollable threat, grassroots organisations like Parkrun are a complement to their own work, bringing millions of people into the sport and helping them to reach the parts of society they could never dream of engaging. “We don’t have the monopoly of running,” the IAAF’s head of road running Alessio Punzi said last year. “Any effort that creates new runners should be cherished. If Parkruns work fine, then let them be.”