The free run every Saturday

At nine o'clock on a cold Saturday morning, about three hundred people are standing in a park in London. Some are serious runners in expensive shoes. Others are pushing babies in buggies or walking with their dogs. In a few minutes, they will all start the same five-kilometre run, and none of them will pay a penny.

This is parkrun. It began in 2004 with just thirteen runners in Bushy Park, in south-west London. The idea was simple: a free, timed 5K, every week, organised by volunteers. Over the years, it has spread to more than twenty countries, and millions of people have taken part.

What makes parkrun different is that it is not really a race, at least not for most people. Instead, it is a weekly meeting for everyone, whatever their speed. There is even a volunteer called the "tail walker", whose job is to finish last so that nobody is ever left behind.

For many people, the social side is just as important as the running. After the run, groups go for coffee and chat about their week. Some doctors in the UK now recommend parkrun to patients who feel lonely or low. This is because regular exercise and friendly faces can both lift your mood.

My own first parkrun was not a success. I bit off more than I could chew: I started far too fast, ran out of breath after one kilometre and walked most of the rest. Nevertheless, a stranger ran beside me for the last few hundred metres and cheered me on. My time is still nothing special, but the run has become part and parcel of my weekend.