LONDON — On a weekend morning in the fall of 2004, 13 runners lined up in a London park for an informal race — unaware they were taking the first steps in what would become a global movement.

Paul Sinton-Hewitt had simple ambitions: to provide a free, weekly 5-kilometer (3.1 mile) run open to anyone.

“I didn’t know who was going to join me on that day,” Sinton-Hewitt said. “I didn’t care how many people came. I would be on the start line every single week for the rest of my life and I would help people to run.”

Parkrun — as it became known — has far exceeded any vision he had, marking its 20th anniversary on Saturday with runs now held in more than 2,500 locations, including 25 prisons, in nearly two dozen countries. More than 10 million people have participated in at least one parkrun and the organization has recorded more than 100 million finishes.

“We’re the smallest that we will ever be,” Sinton-Hewitt said. “In 20 years time when we come back and have this chat again those numbers are going to be minuscule. So it’s a bizarre thing.”

The feel-good fun run is credited with changing countless lives, getting people up and moving, motivating them to come back week after week and nurturing lifelong friendships. Testimonials have come from couch potatoes to people who reversed diabetes and stopped drinking to inmates who found an escape while serving time.

The World Health Organization has endorsed parkrun for offering an accessible way to be physically active and more than 2,000 doctor’s offices have partnerships with the organization to promote its health benefits.

More than 45,000 people who registered for parkrun this year in the U.K. said they had been completely inactive before signing up, according to a study published Tuesday in PLOS Global Public Health.

Of nearly 550 newly registered runners studied over six months, Sheffield Hallam University and the University of Sheffield found their life satisfaction increased after as few as two parkruns.

“All of us say it’s changed our lives,” said Caroline Noon, who started running around age 50 and has completed more than 320 parkruns.

It’s a refrain Sinton-Hewitt hears constantly as the organization’s executive director.

Had it not been for a springer spaniel named Tim, none of this may have happened.

Sinton-Hewitt, a lifelong runner, was training for a marathon when he tripped over his dog and was seriously hurt.

He continued to reinjure himself and he realized that his sub-2:30 marathon dream — once within reach — was dead and his competitive running career was probably over at age 44.

It couldn’t have happened at a worse time. He had lost his job, had relationship problems and was struggling with his mental health. Running had been his outlet since he was a ward of the state as a child growing up in South Africa and now he was sidelined.

Sinton-Hewitt missed the social aspect that came after running and wanted to give back to the sport. He came up with the idea of a 5K time trial where the runners would challenge themselves against the clock and he could join his friends for coffee afterward.

The first run was held in Bushy Park, once the hunting grounds for Henry VIII and later home to the headquarters for Gen. Dwight Eisenhower’s D-Day invasion planning in World War II.

Parkrun didn’t initially take the running world by storm.

After its modest start, the second week drew one additional runner. It shrank to a dozen on the third week. But on Christmas, which occurred that year on a Saturday, there were 25 runners. It was off and running.

As it grew in popularity, Sinton-Hewitt resisted pressure to recreate the event elsewhere until 2007 when a friend presented a plan to organize a second run in Wimbledon. Once he realized it was doable, it quickly expanded that year to six places in the U.K, including Leeds and Brighton, and Zimbabwe, the first international location.

“I never wanted to sell the concept of, ‘Here’s something that you should come and do,’” Sinton-Hewitt said. “I wanted it to sell itself, and it did that because people inherently felt it was good.”

Despite once being labeled a time trial, organizers of parkrun — yes, it’s all lowercase — are adamant that it’s not a race. Participants can run as fast or slow as they want. Many walk the course.

“One of the big barriers that we see from people registering but not yet participating is this misunderstanding that it’s a race, that people won’t be fit enough or fast enough,” CEO Russ Jefferys said.

The organization said it sought to remove those hurdles by taking record times for men and women from its website. Some saw the move as reaction to criticism of its inclusive policy allowing participants to register by the gender of their choice.

Policy Exchange, a U.K. think tank, took aim at parkrun and other athletic events using gender identity, saying three female parkrun records were set by biological males. It wanted parkrun to collect information on participants’ biological sex and update course records or have its government funding stripped.

The organization refused, leaning into its inclusivity and mantra of “free forever for everyone.” Jefferys said that removing the records had been a long time coming.

With a running boom that began during the pandemic, the plan is to continue growing overseas to meet high demand, Jefferys said.

Lithuania became the 23rd country to offer parkrun two weeks ago in its capital, Vilnius. Uganda, Portugal and Switzerland are likely to be next, Jefferys said.

The organization is a U.K.-based global charity, but it also receives public funding in Britain, Ireland and Australia, and also has commercial sponsors.

On a recent Saturday in Hampstead Heath, a wild mix of rolling meadows and woods in north London, Noon was one of nearly 500 parkrunners at the heath, a five-fold increase from when she started in 2014.

“My ambition was only ever not to stop,” she said of her initial parkrun. “I stopped the first time. … I just assumed I’d not be embarrassed and I’d walk home. But I kept coming. Now it’s been 10 years.”