Class conscious - Andrew Martin joins the Hampstead Heath hearties

On Hampstead Heath, you will see lots of stringy-looking folk with nut-brown skin

I walked into Davidoff of London, the tobacconist's in St James's Street, where I bought a Havana cigar. In response to the salesman's habitual question, "Do you have a humidor, sir?" I was able to say "yes", my wife having bought me one for Christmas. It is, essentially, a beautiful cedar box containing a device that holds a sponge full of water. This I have supplemented with an egg cupful of water, because the relative humidity was reading only 65 on the hygrometer, when at least 70 is required. The instructions say that the cigars on the top level should be rotated with those on the lower level. Because I have only one cigar in the humidor this question does not arise. I simply turn it over every so often.

That single cigar justifies the humidor's existence, and for this reason, I may never get round to smoking it, which would suit my wife fine. She actually wants me to stop smoking cigars, and only bought the humidor because she couldn't think of anything else - which is a bit like shooting somebody at a drinks party because you can't come up with any small talk.

As I unwrapped my gift on Christmas morning, she rather feebly said, "Perhaps you'll be able to keep your letters in it." Letters? Does she mean all those brochures I get asking me to join American Express?

My aim, vis-a-vis the humidor, is to smoke fewer and better cigars. For some time, I have been trying to live a healthier - meaning more middle-class - life. My shirt always seems to be untucked these days, and this is because I've lost about a stone in weight over the past few months, mainly by walking fast over Hampstead Heath every day. My sons and I play football at a cluster of seven oak trees described, by a plaque set into the ground, as Denzil's Copse. It was planted 30 years ago in memory of a man called Denzil, by people who were presumably unaware that two of the trees are exactly the right distance apart for goalposts, and that the plaque is ideally placed for a penalty spot.

You see the same people every day yomping across the Heath, and they are not the north Londoners you see driving fat 4x4s. The Heath walkers represent old north London: stringy-looking characters, with nut-brown skin. They're socialists, humanists and, I often suspect, naturists. If your picture appears in the Guardian - as mine has on a couple of occasions - their normal self-contained briskness is checked slightly as they walk past you in the days following.

These people get involved in causes, and their big cause at the moment is the saving of the swimming ponds on Hampstead Heath, which are threatened with closure by the Corporation of London. This does not surprise me at all. I had long since identified the ponds as doomed, simply because I enjoyed them so much. This, I would think, floating along on my back under the trees, is too good to last.

But the most interesting strain of Heath swimmers are the tough old working-class blokes whom you might see shadow-boxing furiously in the cold showers. One of them once said to my elder son, "Many a famous fighter's stood under that water, lad." They speak like characters in Pinter, and read the racing pages between swims - because they'll have more than one in a session. They're the hard core of the year-round swimmers, breaking the ice if necessary or swimming out in fog, far beyond the sight of any lifeguard. Their skinny bodies are often strangely marked, and I wonder whether I detect decades-old bullet wounds.

These men are role models to me, in that I believe them to be fit and healthy not because of some fiddly faddism about what they eat and drink - the modern, colour supplement-reading way - but by what I am convinced is the surer way: simply taking your body, and using it.