On Saturday at 9am, about 80,000 people – more than twice the number who take part in the London Marathon – will gather in parks across the country to run, jog and walk five kilometres. Parents with buggies, children and grandparents, super-fit sprinters, dog walkers and people with disabilities, as well as volunteers, will come together at hundreds of courses across the UK.

The reason tens of thousands of people will forsake their beds for a trot round the local playing field is that they will all be taking part in Parkrun, a series of free, weekly events that, despite the name, are as much – if not more – about community as running itself. The idea has spiralled from 13 runners in Bushy Park in south-west London 11 years ago to a community of more than two million people in 11 countries today, and is expected to double in size in 2016. Parkrun has been the catalyst for marriages, tattoos and millions of friendships as well as mud, sweat and personal bests.

The first parkrun took place in 2004. Paul Sinton-Hewitt, 55, an IT worker who ran for South London Harriers athletics club, decided to set up a different kind of event after tripping and falling during a run while representing his club. A hernia and torn hamstring left him unable to race. “All of a sudden, you're the person who comes to the club to see your friends while everyone else goes running – and the last thing I wanted to do was sit in the clubhouse, waiting to socialise,” he says. “I'd been the recipient of lots of people's goodwill, and decided it was time to give back. With parkrun, I could get my mates to come and see me every Saturday, which was selfish – but I could also give something back to the community.”

By volunteering to hold a timed run each week where people could socialise with the incentive of being active, Sinton-Hewitt felt that he was giving back to everyone who had supported him running over the years. The fact that people could use the run to improve their times, if they wanted, was an added bonus.

Parkrun is different from traditional running and fitness clubs because it is totally free – both financially and of obligation. It takes place at the same time in the same place all year round, and people are free to opt in or out as they wish. There is no pressure or dress code, and runners only register once to participate at any of the 785 events worldwide. Because parkruns are timed runs, with a family atmosphere, rather than the slightly more pressured races associated with a more conventional running club, I feel welcome to join, despite knowing my pace is unlikely to impress.

Sinton-Hewitt chose a distance of five kilometres because it was achievable for beginners while still being challenging for more experienced runners. The recent rise of couch-to-5k programmes has seen many people graduate to parkruns. “I'm surprised something so simple and obvious has become a runaway success,” he says. “But it's free, it's community-owned and it speaks to the things people really want.”

The ten best fitness apps Show all 10 1 /10 The ten best fitness apps The ten best fitness apps Men's Health Workouts With 23 pre-loaded workouts and a new exercise plan every month, this app from fitness bible is an impressive resource if you want to build some serious muscle – not least thanks to the workout The ten best fitness apps British Military Fitness There's no shortage of pedometer apps available, but iTreadmill is a smart choice. It offers options from step counts to working out your average speed whether you're on a treadmill or just out for a walk The ten best fitness apps iTreadmill There's no shortage of pedometer apps available, but iTreadmill is a smart choice. It offers options from step counts to working out your average speed whether you're on a treadmill or just out for a walk The ten best fitness apps Authentic Yoga with Deepak Chopra It's always better to see a real yoga teacher rather than a virtual one, but if you need some hints at home, this app is a useful one. Using photos, audio and video, an instructor demonstrates every routine for you The ten best fitness apps Buddy Runner This free app is great for both runners and walkers. It reports your results (how much distance you've managed to cover, how long it's taken) back to you as you listen to music (unlike some applications that make you stop your tunes before reporting back) and a slightly alarming setting allows you to automatically send them to social networks, such as Facebook and Twitter so you can let your mates know how you're getting on The ten best fitness apps Softrace Get competitive with this beta app that lets you set up real races and go head to head with friends. Participate in several different sports, whether you are on foot or even on skis, and check out your routes and progress on Google Maps The ten best fitness apps Lose it! A free food diary that lets you record what you're eating and when, Lose it! also lets you set weight goals and will remind you if you haven't logged a meal. A helpful way to stay on top of your diet The ten best fitness apps Gym Goal ABC Offering 52 workout routines that span over four levels, Gym Goal ABC gives you step-by-step instructions as well as tips on how to improve your performance. You can add your own exercises, although with nearly 300 already installed, you might not need to The ten best fitness apps Cardio Traine Cardio Trainer provides you with a free personal trainer that fits in your pocket. It uses GPS to show you where you are, and saves your routes automatically so you can refer to them later. It includes a calorie counter and a neat 'race against yourself' feature The ten best fitness apps Map My Fitness Workout Trainer MapMyFitness is a fitness tracking application that enables you to use the built-in GPS of your mobile device to track all of your fitness activities

He hopes to persuade the Government that an emphasis on sportsmanship is not necessarily the best way to help everyone to become active and healthy. “To be a sportsman or sportswoman in a club, you have to either be good at the sport, or oblivious to the pressures that come with being in a club,” he says. “Sport isn't the way to measure health: if you walk to work, that should count, too.”

His beliefs are reflected in the ethos of parkrun. While each event is timed so people can track their speed – and the barcode technology generates thousands of results each Saturday – volunteers stress that this is a run, not a race.

Anyone can enter for free: parkruns rely on funding from commercial sponsors and the goodwill of volunteers to operate. Sinton-Hewitt, who worked full-time in IT until 2010, initially using half his income to fund parkrun, says: “Now we have commercial sponsors and sometimes the Government helps. We generate £3,000 to start up each event. We're very careful not to over-commercialise – there's a balance to be maintained.” Parkrun employs 20 paid staff across the UK and nine more worldwide.

Anyone can sign up to the parkrun website and print out a barcode. This code is used to record runners' times and can be used each week at parkruns held anywhere in the world. For those who are competitive, parkrun semi-reluctantly notes the “first finisher”, rather than winner. Reassuringly, you can never come last (“final finisher” wouldn't mask it), because there is a volunteer keeping pace at the end.

And while there are hundreds of professional athletes who parkrun, there are also hundreds of thousands of people who turn up and take part but who never would have considered running before.

“When we start at each new location for Parkrun, we initially focus on people most likely to take part,” Sinton-Hewitt says. “Then others feel safe and unobtrusive in joining the community.” At Bushy Park, almost 1,000 people gather each Saturday: about half are typical runners, while the others would have never considered jogging round the park without the pressure-free framework of parkrun. There are also a growing band of walkers, people in wheelchairs and joggers who take part in the parkrun routes.

I join the Hilly Fields parkrun, in Lewisham, south-east London, on a Saturday morning in December to find out why so many people are being persuaded to trade lie-ins for exercise. Emma Malcolm, 44, a charity worker for Rethink Mental Illness, is organising today's run. She is one of the 10,000 volunteers who help out each week, setting up the course, giving a safety briefing to runners, scanning runners' barcodes, marshalling on the course, processing results and writing the run report for the website. As a run director, she volunteers two weeks out of every 10. The events have become so important to her that she planned her holiday in the US around Parkrun, so she could run courses on the other side of the Atlantic.

Today, she sets the course and gives the run briefing before we start – pointing out the direction we take. She tells me that Hilly Fields is one of 78 courses across the UK hosting junior parkruns for four to 14-year-olds, held each Sunday and covering two kilometres. Children from the age of five can also join the main Saturday parkrun, although they must be accompanied if under 11.

Emma is joined by a team of volunteers including John Barron, 60, from Blackheath. He joined parkrun in 2012 and has completed 130 runs. When he isn't running, he volunteers – as many regular runners do – and is so enthusiastic at cheering others on that he's a big deal round Hilly Fields, credited for many regulars' record runs.

Five years ago John had a heart attack, and decided to change his lifestyle. He now does five kilometres in 21 minutes and 10k in less than 43 minutes, and runs for Kent Athletics Club vets team. He loves the social side of parkrun. “It's given me new friends of all ages,” he says. “And I've reduced my statin dose, which was causing me problems.”

But he warns me: “This is one of the hardest courses. Lots of people come from other runs to improve their times on their home course.”

As we set off, I regret not having been running since the summer – and even then I was only going for little jogs. I also regret having not picked up the clue in the name Hilly Fields. I am swiftly overtaken by a man with a pram. I can hear my breaths draw heavily, but each time I consider halting, a volunteer is shouting encouragement and it keeps my feet moving.

As I complete my last lap around the park, and approach the biggest hill, I slow down on the treacly incline. John Barron springs into action, running up with me, telling me I can do it. His encouragement carries me the last paces to the top and around the corner to a running finish. I come in at 113th – towards the back of the pack at just over 30 minutes – but I'm elated to have completed the course and am determined to come back and improve.

I join crowds who have drifted into the park's café to get drinks. Since the first parkrun 11 years ago, cafés close to each event have become part of parkrunners' Saturday culture.

Lisa Power is drinking coffee with her father, Sean, 68, and five-year-old son Joseph. They live locally, and Joseph has done junior parkruns 30 times. Lisa started running in 2013. “I got into it to escape my children,” she says. “They love it too. I get half an hour to run to myself. The camaraderie is incredible: within six months of parkrunning, I knew more people in London than I had in 10 years of living here. I see my progress because it's timed; I have achieved something by 10am on a Saturday; and now, when I walk down the street, I find I know people.”

Her enthusiasm seems to be shared by everyone in the café. Tien Wilde, 42, from Lewisham, has run more than 100 times. “No competition,” she says, “It's just against yourself.” She took part in a zero-to-five kilometre running course after having her son Noah, now six. “He's done 12 parkruns now, and he's faster than me,” she says.

Margaret Glover, 54, comes to parkrun with her husband Eric, aged 60. “PE lessons were dreadful when I was at school, and it put me off exercise.” she says. “This has turned me around. People are encouraging, even if you're not good, and it all makes a difference.” She walks briskly as she has a bad knee, clocking a time each week of around 43 minutes.

Each year, every parkrun has the option of holding extra events on Christmas Day and New Year's Day. At Hilly Fields last New Year, a runner had a heart attack. Two doctors running the course helped first-aiders keep him alive until an ambulance came. They saved his life, and he continues parkrunning in Peckham. The British Heart Foundation provided a defibrillator that now hangs inside the door of the café.

Sinton-Hewitt says that there have been nearly 12 million runs recorded, and 20 incidents where people's lives have been saved by parkrunners. There have been four unpreventable deaths in this country. “We have a policy that allows for parkrunners to record if they are up-to-date registered first-aiders,” he says. “We are looking at running a first-aider programme to train people for free, which we think will revolutionise first aid for running events in this country. We could provide the backbone for most small events – it would be groundbreaking.”

The community I find at Hilly Fields is, I'm told time and time again, replicated across the country – and with it the lows and highs of life. In the past six weeks alone, there have been three weddings of people who met at parkrun. Arlene Gallagher, a 36-year-old civil servant who organises Gunnersbury parkrun, is about to marry her partner Brian Stakelum, who she met at the event.

“Brian had never been to a parkrun before, so came to ask how it worked. He then ran – and came second or third out of everybody, which surprised me,” she says. “He looked me up on Facebook, and we agreed to go for coffee, which ended up in the pub.” Brian proposed in June and Arlene held her hen party at a parkrun, with a group of women dressed appropriately for the occasion.

David Mushet, 53, from Paisley, runs the Greenock parkrun in Scotland. He had a liver transplant at the beginning of 2013 after a debilitating illness. “I will be eternally grateful to the donor,” he says. “After the operation, my physiotherapist encouraged me to walk. It took me 30 minutes to walk 100 yards. But I got fitter and fitter, and walked and walked.”

His physiotherapist told him about the British Transplant Games, which has been running for the past 30 years, and he joined the Edinburgh team, winning the gold medal in the 5k race with a time of 42 minutes. He was picked to represent Great Britain in the World Transplant Games, held in Argentina, and came sixth in the world. He can now walk the course in less than 35 minutes, has taken part in more than 40 parkruns and has volunteered 10 times. He hopes to be picked for team GB again. “I'm doing things I never dreamt of,” he says.

There are also tourist parkrunners, who cover as many different courses as possible. The current record holder is Paul Freyne, a 44-year-old credit manager for Royal Bank of Scotland, living in Twickenham, west London. He started running in 2003 to lose weight. “I'm a rugby-player build,” he says. He has run 241 parkrun courses in the UK and a further 34 elsewhere in the world, in countries such as Poland, Denmark, Russia and the US.

His favourite courses are in Ireland and Northern Ireland: the former is on Bere Island, a ferry ride off the south coast near Cork; the latter is at Portrush in County Antrim, a kilometre from the Giant's Causeway, along the beach. At Portrush, everyone runs two and a half kilometres each way by the sea. “It's a beautiful bay, and you see horses galloping along the beach on the softer sand, while running on the firm sand by the waves,” he says.

Vertical running

It has also inspired other events. Tommy Hill, 29, was introduced to Parkrun in 2011. “I fell in love with it,” he says. “It was amazing – although not physically, as I hadn't really exercised from the age of 16 to my mid-twenties, so there were times during the five kilometres when I walked.”

He went on to set up Hanley parkrun in Stoke-on-Trent in 2011, while he was a student. To set up any new parkrun, the founder or team must get formal permission from land managers to use the location each week, and raise a one-off fee to Parkrun of £3,000. This is done through councils, public-health grants, country sports partnerships or local running clubs. Volunteers are supported by parkrun HQ as they go through the process, and no money needs to be raised in the future as commercial sponsorships mean each event is self-sustaining once launched.

The experience gave Tommy the confidence to found the Birmingham Swifts, the first LGBT running club in the area, when he moved to the city two years ago. “I wanted to join a running club aimed particularly at the LGBT community, because I wanted the social aspect as well as the running side. I couldn't find one, so set one up myself. I had the confidence from setting up Hanley parkrun,” he says. The club now has 50 members of different ages and cultures, who all want a safe place to run.

Sinton-Hewitt expects Parkrun to continue expanding, with more than 1,000 events worldwide predicted by the end of 2017. In the UK, new parkruns spring up almost weekly. Overseas, he will focus on the US, where some local authorities have been less supportive than in the UK, trying to charge Parkrun to operate – with Boston asking for $30,000. He says he will then focus on China. There is no doubt that parkrun has transformed many lives – Sinton-Hewitt's included. “It's taken 12 years of very hard work, but I've never been quite as content with life as I am now,” he says.

And while my time round Hilly Fields was slow, my cheeks red and my legs sore, I felt happy. The inclusiveness of parkrun – welcoming everyone for free and run by volunteers – makes it feel uncynical. It's a refreshing way to start a weekend. If you're going this Saturday, give a wave to the stragglers: I'll be among them.