It was a mildly rainy day when I met the Bermondsey Bombers. I was on tenterhooks. "They'll go anywhere," my informant Sylvia Livett had said, approvingly. Starting off as a small tentative group, they had now got the bit firmly between their teeth: parks, museums, towpaths, bridleways – the Bombers were up for it.

Sylvia's role within the Southwark Primary Care Trust is to get walking groups going – whether in green spaces or city streets – as part of the national Walking for Health movement. If you're part of a group, she'll argue, you are more motivated and more supported. And as a by-product you will be healthier.

Walking in a group isn't a new idea. The Ramblers, set up 76 years ago, are the seasoned denizens of walking for pleasure, and their 12-week Get Walking Keep Walking scheme (getwalking.org) is aimed at people you wouldn't normally find on a ramble. What is new is the direct link with both physical and mental well-being, backed up increasingly by research.

It was this, plus his own experience, that led William Bird, a country GP near Henley-on-Thames, Oxfordshire, to set up Walking for Health.

It had started when a series of patients passed through Bird's surgery, all apologetically admitting that they hadn't taken their medication or followed a recommended regime. Gloomily, he went for a twilight walk with his dog. As he tramped he looked beyond the fields to lighted windows in the homes of his recalcitrant patients. Their treatment was, he thought, on their doorstep, if they'd just look out and see it …

The thought followed him back home, where he started to draw up a series of walks. He even persuaded a friend with a light aircraft to take him up for £50 to photograph the terrain. Finally, he laid his maps out in his surgery. They were taken up, he said ruefully, but only by people who were walkers anyhow.

Why were the ones he was aiming at not tempted? He called a meeting in the village hall. And there Bird realised what was needed: "They said as one, 'We want to be led!'"

The first Walking for Health guided walk took place in April 1996, immediately spawned 10 more, and earned Bird the Innovation Doctor of the Year award from Doctor Magazine. Since then the scheme has grown, come under the wing of Natural England and, with the involvement of the British Heart Foundation, gained a sizable Lottery grant. Now the Ramblers and Macmillan have taken it over. Around 75,000 people of all types, ages and conditions now walk together every week, following volunteer leaders.

On the streets of London, it all feels more homespun. I am waiting for the Bermondsey Bombers in faint drizzle at a bus stop near the Tower of London. Beside me, 39-year-old Klair, who has been leading the group for three years, is checking her gear: map, first-aid kit, step-counter, insurance. Dressed in a fluorescent security vest, stout boots and the kind of hat you'd expect to see in the Australian outback, she is a formidable figure.

Like many leaders, Klair came up through the ranks. She'd joined her first walk reluctantly, to counter diabetes, depression and a serious weight problem, little suspecting what lay in store. Then the three Bombers tumbled off the bus, and their energy and jollity was palpable.

"We love Klair!" said 67-year-old Yvonne, a retired hospital worker. "Mind you write that!"

Klair, who is a round-faced, reticent woman, beams silently. But as we set off at a steady amble round the Tower, she tells me that being part of the scheme has changed her life – she's three stones lighter, far happier and her diabetes is in check.

We are a small group of Bombers that day. There had been heavy rain earlier and a number didn't make their way up from Bermondsey. But it's a gossipy chatty group that wanders along the Thames, past a mortuary built, we learn, to house drowned bodies, and down quiet cobbled backstreets to Wapping.

"It's got to be enjoyable," Bird had stressed. "If people want to stop for a piece of cake, that's fine."

I suspect sociability is the vital factor: it keeps Yvonne, Gloria and Andy – and the dozen or so other regular Bombers – coming back time after time, they all agree, as Victorian London unrolls itself around us.

"It's not a marathon," insists Klair. But nevertheless when we get to Wapping and she proposes carrying on to Canary Wharf, I peel off. The Bombers have more stamina than me and have walked me off my feet.

Walks are graded, and are open to all. Their genesis may be medical, and Bird is passionate about the beneficial impact on stress, heart disease, diabetes and in protecting against various forms of cancer. But most of the walkers, Natural England emphasises, do not join because of a health problem.

I ask Klair what the secret ingredient is. Before I leave, she produces a text which is addressed to me. "I'm sorry I can't be with the walkers today'' said the anonymous writer, "but I would still like to tell you about my experience. I was introduced to the Bermondsey Bombers by chance. At the time I was recovering from the death of my husband and the loss of my job and the main part of my social life, which all happened at the same time. I was at a very low ebb and had spent more than a year sitting at home in a deep depression. It is no exaggeration to say that joining the Bermondsey Bombers has turned my life around. To be honest, the regular, gentle exercise is a small part of what the group provides; far more importantly it offers non-judgmental friendships, support and fellowship. I feel the group as a whole is developing and a huge debt is owed to our leader, Klair, who turns out every week, no matter what the weather, and looks after us discreetly but like a mother hen! She is wonderful."

So there you have it – the secret ingredients: company, exploration, regularity, kindness. And don't forget the cake.

• A new partnership, the Ramblers and Macmillan Cancer Support, have taken over Walking for Health. For further information and to find one of 600 free local walk schemes across England, see walkingforhealth.org.uk. The Ramblers also run led walks under the Walking for Health umbrella in Birmingham, Durham, London, Manchester and Sheffield – see getwalking.org