Shape up the John Rentoul way! A seven-step fitness guide

He's a 6ft 2in professor of history and not exactly a keen sportsman. Yet he weighs in at just 10 stone. What then is our political commentator's astonishing secret?

It was my fault, really. The editor, who runs marathons and things, said that all of us middle-aged men know about the problem of spreading waists. As everybody else nodded in agreement, I dissented. "Well..." I said. To myself, really, I thought. But once you have said it out loud in an editorial meeting, it is too late. Just because I am 6ft 2in and weigh 10 and a quarter stone and think that a BMI is a kind of stunt bike, some of my colleagues seem to think that I might have some advice for them. As it happens, I have, but I'm not one for blowing my own trumpet. However, as the trumpet is now out of the trumpet case, as it were, I might as well give it a toot. It is, after all, that time of year. That is, the start of a new one. A time for fad diets and exercise regimes and women's magazines going on about the new you and the miracle this and the macrobiotic that. So, here you are: this is how I maintain my alluring physique, and how you, too, could have figure just like mine.

Play tennis

It is essential to wear the right kit. You should wear braces, because otherwise your trousers will fall down, and a jacket is advised when it is cold or looking like rain.

As you grow older...

... it is more important to keep the brain active. Some do Sudokus but I recommend marking essays. This has become even more challenging in recent years as these are the only times when anyone over 18 uses handwriting.

Weight training

Is highly fashionable, I believe. I recommend the regular lifting of a heavy book, especially any volume of Robert Caro's The Years of Lyndon Johnson. Pay particular attention to the angle of the lift, however. Use your legs to tilt the whole body to avoid back strain.

A healthy diet is essential

Brain work requires a lot of energy and it is important not to allow your blood sugar levels to drop, especially at vulnerable times such as mid-morning and mid-afternoon. Carrot cake has got carrot in it.

Do not go to a gym

It is expensive and you waste a lot of time travelling from A to A. An actual bicycle will take you to B, the seminar room, or even to C, the Sky News studio. Note: checking emails while cycling is bad for your health.

Cheerleading

Is an accepted sport in many schools these days. If I feel the need for resistance training, I go out for an invigorating spot of defending Tony Blair – "on balance a good prime minister" – against the irresistible force of conventional wisdom.

Modern Life

It is essential to keep up with the hectic speed of modern life. I advise taking to Twitter whenever you have a spare moment. If you catch yourself staring out of the window with your mouth open, reach for the iPad and be really witty and clever. If you can't manage that, just be rude to someone and then pretend that Twitter fails to convey irony.