Paul Sinton-Hewitt had reached a crossroads in his life: sacked from his marketing job and having had an injury that ended his marathon hopes, he organised a run around his local park in London one Saturday, timing his friends’ efforts before they all went for a cooked breakfast.

That meeting of 13 runners for the first parkrun has since morphed into a global phenomenon. This week, parkrun celebrated its 15th birthday, with 6 million people – from prisoners to pensioners, buggy-pushers to record-breakers – now signed up to the community event and 4 million having participated.

Every week, more than 350,000 people take to their local green space for the free, timed 5km (3.1-mile) parkrun, expecting to reap the health benefits.

After its creation in 2004, the first parkrun outside the UK was started in Zimbabwe three years later. In April, Japan became the 21st country to host the event, launching a Tokyo parkrun.

Meanwhile, parkrun Russia started with two simultaneous runs in parks in Moscow in March 2014, and there is now an event in Siberia, where organisers have pledged never to let snow, ice or cold weather force them to cancel.

Paul Sinton-Hewitt, parkrun’s founder, said: ‘One of the things that I’m most proud of is that every year the average finish time has slowed.’ Photograph: parkrun

In Africa, runners in Eswatini (formerly Swaziland) and South Africa’s vast cities and grasslands are also regulars.

The combined distance so far is 162m miles (260m km) at 53.8m parkruns globally, which adds up to 3,118 years, 25 days, 22 hours, 7 minutes and 18 seconds of running and walking.

On reaching the milestone, Sinton-Hewitt, who was appointed a CBE for services to grassroots sport participation, said: “When I invited my friends to join me in Bushy park [in south-west London] back in October 2004, never in my wildest dreams could I have imagined more than 4 million people would have taken part 15 years later.”

For many, the event, entirely organised by volunteers within communities, has changed their lives.

Dawn Nisbet from Oldham, who recently completed her 70th parkrun and is a run director for the event, said: “In April 2016, my stepmother was diagnosed with ovarian cancer. I was also very overweight, completely inactive except walking my two dogs. Being so unfit and unhealthy, my opinion of myself was low, which limited a lot of my activity and self-confidence, but then a friend suggested parkrun.

“I’d never heard of it before as, quite frankly, it’s not something I would have ever considered as for me. I researched it on the internet and sent an initial email – I think if I’m honest, I was hoping they would say, ‘No, you’re too slow and don’t run the whole 5k, so thanks but no thanks’. But they were wonderful, so welcoming, happy for me to walk the whole thing or whatever suited me.”

The NHS assistant director, who has since completed four 10km runs and a half marathon, said she was drawn to the event due to the lack of hierarchy and intensity normally associated with mass participation sporting events.

“It’s not about coming first or winning. You can go at your own pace and just enjoy being with other people. It is so inclusive and the perfect way to start your weekend in the outdoors chatting to people. It has completely changed my outlook on life,” said Nisbet.

Of the almost 2,000 parkruns held each week in 21 countries, 25 take place in prisons – 18 in the UK, two in Ireland and five in Australia – where prisoners and staff often complete multiple laps of an exercise yard to reach 5km.

Next week, parkrun will reach another landmark by launching its first prison event in the UK for women only, following the lead of Australia.

The fastest recorded time at any parkrun was set in 2012 in Bushy Park by the British Olympian Andy Baddeley, who finished in 13 minutes and 48 seconds.

Charlotte Arter set the best women’s time of 15 minutes and 50 seconds in Cardiff in January. Notable participants include the Olympians Jessica Ennis-Hill and Dame Kelly Holmes.

But parkrun organisers, who are keen to avoid putting people off with competitive comparisons and are delighted for people to walk the course, do not have a record of the slowest time.

Sinton-Hewitt said: “One of the things that I’m most proud of is that every year the average finish time at parkrun has slowed. That means we are reaching more people for whom physical activity hasn’t been the norm.

“I would like to see that continue, and for more people to understand that parkrun is truly welcoming of those who want to walk, jog, run or volunteer, especially those who would like to take their first steps on the path of being physically active.”