Sitting at computer screens or in theatres is not the best way to keep healthy, as I discovered the hard way 20 years ago when suddenly afflicted with a severe case of repetitive strain injury, not to mention heavy breathing of the non-sexual kind.

The latter was the result of a curious non-diagnosed asthmatic condition which means that if I set off on a run, or a jog, I'm less breathless at the end of it than I am at the start. And those runs began as a way of dealing with the RSI, which is basically a case of your blood not circulating properly.

So, just as we start filling in our diaries and planning ahead, we are assailed on all sides by fitness supplements in the newspapers and online invitations to shape up, stretch those tummy muscles and reach for the stars.

You can ignore the lot of them and do just three things: eat less, forswear alcohol (it's all sugar) for a couple of months, take exercise. The latter could be just half an hour's walk a day, but even hopping on a bus is better than sitting in a car.

I thought smugly about all of this as I drew heavily on my first and last cigar of the year after last week's performance of Fuerzabruta (Brute Force) at the Roundhouse. Earlier in the day I had registered my participation in the Stratford-upon-Avon half-marathon at the end of April, and the finishing line in the field across the river from the theatre was already in sight.

Of course, promenade theatre might count as exercise in some people's books, and you can move around as much as you like during Fuerzabruta, a show that is noisy, exhilarating and totally vacuous. And I dare say some folk might consider that participating in You Me Bum Bum Train is the equivalent of a good work out in the gym.

But the real attraction of exercise is surely the fact that you do it, or should do it, in the open air, and preferably not running through a lot of car fumes and exhaust pipes. Sometimes this is unavoidable, as on the day shortly before Christmas when I was diverted from Hampstead Heath to deliver a few Christmas cards by hand in Belsize Park and therefore ran down Rosslyn Hill.

As I sped, well, staggered, past the Everyman Cinema, I was assailed by that familiar, tedious cry of, "You shouldn't be doing that, jogging is bad for you," always emanating from the over-plump, ragged and super-unfit. My heckler was Micheline Wandor, poet and playwright, former wife of uber-agent Ed Victor, and author of a long-ago feminist riposte to John Osborne, What About Janey Porter? I kept going, shouting seasonal greetings as she struggled miserably (I nearly said "un-manfully") around with her shopping bags.

A few days later I bumped into choreographer Arlene Phillips on the same parade of shops, but as we were both inside a supermarket, I could engage her briefly in conversation and congratulate her on her CBE in the honours list. She said she was still in shock, doubly so because she'd been agitating, with others, for a damehood for Gillian Lynne.

As I've slipped into name-drop mode, it may be time for an update on my local celebrity non-watch. I rarely see Melvyn Bragg or Michael Palin jogging on the heath these days, though I know for a fact the latter still heaves his guts over Parliament Hill Fields when not travelling in foreign parts for a BBC documentary. 

The meanest, most dedicated day-time runner around here remains Christopher Eccleston, whom I've never spoken to (he's far too fast for me) while concert pianist John Lill, wonderfully aloof and private, still takes a brisk and sprightly walk between practice sesssions. I occasionally see actress Patti Love and casting director (retired) Gillian Diamond walking their dogs. I once saw Simon Callow with his.

When I turn right and go towards Regent's Park via Primrose Hill, I pass the houses of Gwyneth Paltrow and Chris Martin, David Walliams and (opposite him on the same road) Derek Jacobi within the first few hundred yards, but I rarely see them. I haven't seen Sylvester McCoy for ages - he's touring in The Mousetrap, apparently - and Declan Donnellan and Nick Ormerod must be either working in Russia again or on retreat with the Trappist monks.

It is possible, of course, that all of these people are avoiding me deliberately, or I'm losing my touch and acuity of observation. At the Royal Albert Hall yesterday afternoon - where I enjoyed Cirque du Soleil for the first time in years - I thought Richard Wattis had come back to life when a remarkable lookalike of the hilarious, officious ditherer in so many films and television sitcoms was hauled out of the audience for a bit of participation.

It's curious what five minutes in the spotlight can do for a person. As I made my way back to South Kensington tube station along Exhibition Road - and what a marvellous little walk that is, these days - I spotted "Wattis" again in the milling crowd. I could only just restrain myself from bounding up to him to blubber congratulations on his improvised performance.

But he'd shrunk back into complete ordinariness, unfazed and unrecognised, a non-actor of stunning banality accompanied by an equally ordinary-looking wife (or sister, possibly) with an unattended hair-style, dull trousers and winter socks. And then I began wondering: why had they gone to Cirque du Soleil in the first place? The show could have corresponded with absolutely nothing at all in their lives.

Nor in ours. And that's the answer. We go to the theatre because that's not how we live. But we must try and make sure that how we do live is the best, and healthiest, way possible. Happy New Year!