Twenty years ago, on a windy, autumnal Saturday morning, 13 runners showed up to a park in south-west London for an event called the Bushy Park Time Trial. A 5km course was plotted and the organiser, Paul Sinton-Hewitt, a computer programmer who grew up in South Africa, bought washers from a hardware store to hand out as finish tokens. The times were tapped up on a laptop afterwards in a local Caffè Nero.

This Saturday, the weather hadn’t much improved – overcast, with the sun straining to peek through – and the venue was the same: picturesque Bushy Park with its resident red deer squaring up, ready to rut. But pretty much everything else about the impromptu get-together has evolved. Since 2008, it has been known as Parkrun and there are now 2,500 weekly events – all 5km, all free – in 22 countries, everywhere from the slopes of Mount Etna to 25 UK prisons to the Falkland Islands. In a typical week, around 350,000 people will take part. Runner’s World hails it a “global phenomenon”.

At Bushy Park, just under 2,000 turned up for this weekend’s anniversary. Parkrun’s regular roll call includes Olympians and a 100-year-old man in New Zealand who has notched up more than 175, and everyone in between. On Saturday, the Paralympian swimmer Ellie Simmonds was volunteering at Bushy Park and one of the runners was Richard Fletcher, a local who did his first in January 2006 and on Saturday completed his 800th event.

“Once you get started, once you’re hooked, it’s like a drug, you can’t stop it,” says Fletcher, with a broad grin, after crossing the line. “Every Saturday at 9am, wherever I am in the world, I’m going to try and do a Parkrun. I’ve done one in Kraków. I was in Helsinki, I did one there. I was in Namibia last year in Swakopmund and I just showed up, and the guys were superb. We start, we finish, and we all have a bit of banter at the end of it. It’s a superb community spirit.”

The 64-year-old Sinton-Hewitt, who was awarded a CBE in 2014, was also back at Bushy Park on Saturday, though not running (he has arthritis in his left knee). As well as the physical benefits of walking or running 5km each week, he believes that the reason Parkrun has chimed with so many people is the boost it gives to mental health.

“I’ve always had issues with mood swings,” he says, “and I knew that, mentally, I needed running to keep the boat afloat. So I figured the best thing I could do was start this event and I’d see my mates every weekend. And, 10 years later, it was clear that it had been my secret weapon, my little pill I took every single weekend, which helped. It didn’t fix me, because I’m still flawed, but it helped to address the ups and the downs.”

Recent research from Sheffield Hallam University and the University of Sheffield seems to enforce Sinton-Hewitt’s point. The six-month study, which looked at 548 newly registered Parkrunners, and used the UK Office for National Statistics metrics for measuring wellbeing, showed life satisfaction increased after doing just two Parkruns. “During Covid, life satisfaction went down by 0.4 in the UK population using the ONS data,” says Professor Steve Haake, from Sheffield Hallam University’s Advanced Wellbeing Research Centre. “And after Parkrun, life satisfaction went up by 0.25. So that gives me a feeling for how powerful something like Parkrun is.

“I work mostly in public health,” Haake goes on, “and I can’t think of any public health initiative in the UK as large as Parkrun.”

Parkrun, which became a charity in 2017, has not entirely avoided controversy in its two decades. There were complaints earlier this year – and some calls for Sport England to withdraw its funding – over the fact that trans women are allowed to identify as female. (When runners register, if they don’t want to state their sex, they can specify: “another gender identity” or “prefer not to say”.)

Parkrun has also removed some of its speed records from its website to avoid being “off-putting” to new participants.

Overwhelmingly, though, the regulars love Parkrun and see its ethos – free; open to all; an event not a race – as something rare and worth cherishing.

As the prosecco modestly flowed at Bushy Park, Sinton-Hewitt looked towards the future. “I live with it every single day so there’s no great shock in the new milestones,” he says.

“There’s just a wonder about where we will be in 20 years’ time. We’re only going to get bigger.

“So we’re 10 million people registered now – is it going to be 15 million in 20 years’ time? Or 100 million?”