A colleague answered the phone for me the other day. He left a message saying, 'Russell of Kensington rang - he can't make it tonight.'

For the life of me, I couldn't work out who the caller was. The colleague said he had sounded familiar, as if he knew me. I could not recall any engagement that evening. And then I checked my diary. 'It's Russell, my personal trainer,' I yelled to general merriment in the office.

'Personal trainer' is both an exaggeration and a euphemism to hide my embarrassment. Russell, whom a friend insists on calling 'Sven' because, as everyone knows, all personal trainers are Scandinavian, is manager of a GP referral scheme in Kensington and Chelsea which aims to encourage people with minor but potentially serious ailments to take regular exercise.

Instead of popping a pill, they are prescribed exercise. In my case, water on the knee led my GP to tell me I must go to the gym. It was a bit like the time the consultant looked at the pre-cancerous condition in my mouth and said: 'If you don't stop smoking, I won't treat you.' I haven't smoked since, and I'm kind of hoping the exercise will do the same trick.

The tests on my knee produced no real diagnosis. Yes, there was the dreaded herald of the ageing process: 'wear and tear' to report from the x-rays. But no firm cause for why it had suddenly ballooned up, so that I could hardly walk and only anti-inflammatories could keep the pain in check.

My GP, a forceful woman, told me the only thing to do was to reduce the wear and tear by losing weight and getting regular exercise. She has known me for many years and knows my weight goes up and down, particularly when I'm under stress or depressed. I find alcohol and good food a great antidote to the blues.

I have used gyms before, but not under the supervision of someone trying to help older people, those recovering from heart disease or those who are simply atrophying because of obesity get into the habit of regularly mounting an aerobic bike or using a step machine.

Fortunately, I have no qualms about appearing in old jogging trousers and t-shirts among the Lycra-clad regular users of the gym. So I did not feel the need to invest in new sports gear. But I was concerned when I was told that I would have to wear a heart monitor when I first turned up. Russell neglected to tell me to wear it next to my skin, so I had to hurriedly remove my bra in order to make the necessary adjustment.

Apparently, my heart is in good nick because we dispensed with the monitor after the first session. I was introduced gradually to various pieces of aerobic equipment. Most sessions start with 15 minutes on an exercise bike, followed by exercises and finishing with another 15 minutes on a rowing machine. I take pride in the fact that I regularly cycle 4 kilometres and row more than 2,000 metres.

But I couldn't use the jogging machine because the impact had a bad effect on my knee. I got into the habit of rushing home from work twice a week, feeding the cats, changing and dashing out again for what I can't really call a workout. But each session does work up a sweat, uses at least 200 calories - and has the bonus of allowing me to eat more under the diet I adopted at the same time. Half an hour in a gym equals a small jacket potato plus a glass of wine.

Russell encouraged me to take more exercise, so I now swim twice a week as well. Half an hour in the pool means I can add some baked beans to the jacket potato and glass of wine.

The other day, someone commented that my upper arms were looking good. Today, I easily got into a pair of trousers that has hung in the wardrobe for two years, waiting for me to lose those vital inches.

Forgive me for being so personal, but this is the last of my summer columns. And I thought sharing my experience might encourage stressed-out managers to consider exercise, too. I can tell you that the GP referral scheme I have been using is successful: 85 per cent of those referred have completed the 10-week programme or are still using the facilities opened up to them.