Four years ago, I put on what I used to call my gym kit and went for my first run in decades. Two hundred metres in, pain shot through my ample, but I liked to think, still manly physique. It was as if a sniper had taken out my legs. Abandoning all thought of fitness, I hobbled home, wincing with every step.

One should try to grow old honestly. And for middle-aged men that includes facing up to self-delusions. For anyone brought up in the journalistic culture of the late 20th century, the greatest delusions were about health. All kinds of subconscious diversion strategies clicked in whenever it was suggested that perhaps we should think about leaving the pub occasionally and taking some exercise.

Then, and indeed now, if a middle-aged man announces he is off for a boozy lunch, he is highly unlikely to say to himself: “But I must be careful. What about my liver? What about my heart?” He is even more unlikely to hear his beery friends say the same. They, too, do not want the thought to gain currency that boozing and gluttony may be dangerous.

When you announce you are taking up running, however, your friends turn into health freaks. “No, Nick,” they cry. “Think of the dangers. For God’s sake man THINK OF YOUR KNEES!”

That my knees, and indeed my thighs and calves, had gone within minutes of striding out confirmed what they and I wanted to know. Exercise was dangerous, and it was health fascism to think otherwise. Far better and safer to stay overweight, even though in my case I was obese (weighing about 18 stone or 114kg) and the real danger to my knees came from my forcing them to dead lift 30kg of surplus blubber every time I levered myself from a chair.

Until recently, the debate about obesity paid too little attention to why people stayed unfit. It is not as if we were happy. I know the attraction of getting drunk. It has taken me half a lifetime to break my addiction to nicotine. But why did I live in a semi-paralysed condition where fast movement and vigorous labour were impossible?

It is important not to lose sight of how extraordinary the change to our bodies has been and how widespread the acceptance of debilitation has become. Britain is now the fattest country in western Europe. In 1957, fewer than one in 10 11-year-olds was fat; by 2012, more than a quarter were overweight or obese. Obesity is responsible for about one in 10 deaths in Britain. It vastly increases the risk of developing type 2 diabetes, the main cause of blindness in working-age people, and leads to more than 100 amputations a week.

One good answer to the question why do not we eat less and move more is that our idea of a normal body has shifted. As our size has ballooned, so our expectations have changed. American academics found, for instance, that their overweight compatriots no longer think of themselves as fat. When they compared themselves against the people around them, they thought their bodies looked “just fine”. The same academics found that fewer people are bothering to diet. Many no longer see the need to. Obesity, like the Trump administration, has become accepted as the new normal.

In journalism, the old hard-drinking culture generated all kinds of mental defences against losing weight. Twenty years ago you could earn yourself an air of unwarranted sophistication by quoting Tony Benn’s quip that when the “desire for physical exercise gets too strong, I lie down until it passes”. There were dozens of variations on a line I remember John Mortimer delivering: “I’ve always believed that no pleasure is worth giving up for six months in a geriatric home in Weston-super-Mare”.

Ha bloody ha. Except that when the guffawing stopped, the punchline rang false. Modern men and women are not always dying prematurely. Lifestyle diseases tend to be chronic before they are fatal. Rather than escape the geriatric home, we are condemning ourselves to a sentence of considerably more than six months of miserable infirmity.

There is a further factor in explaining why we don’t run. The British in particular are frightened of looking absurd. And before he is anything else a man of my age in shorts and a clinging top stepping out onto a public highway is utterly ridiculous. Not least because he cannot move far or fast once he is on the road.

This point is too rarely made. The food industry is using its ferocious lobbying power to try to limit restrictions on sugar. In all the propaganda that drops in my inbox, its PRs say that weight loss is as much about exercise as cutting down on the sugar-laden foods they dispense. So it is. But if you are obese, as I was, you cannot take strenuous exercise. You cannot walk five miles without feeling it for days afterwards, let alone run five miles. You have to change your diet before you can do anything else.

Finally, and there is no way of avoiding it, there is a repellently religious aspect to proselytising for exercise. Like repentant sinners at an evangelical meeting, we describe with lubricous relish how low we sunk before we were reborn. We pigged out. We boozed up, but we saw the light and expect our newfound virtue to be applauded. (I’m 13 stone now. Praise the Lord, and more to the point, Praise Me.) Purity of body may have replaced purity of soul but the self-satisfied piety has not changed.

I would not write now about the pleasure I have found in running were it not for two factors. First the National Health Service cannot cope with the sicknesses of obesity on top of the chronic illnesses of ageing. I am speaking with no authority beyond anecdotal experience, but it strikes me that public health authorities need to think about the psychological as much as the physical causes of obesity.

They need to find ways to overcome our bias towards indolence and our willingness to delude ourselves. Second, a doctor who rather sweetly overestimated my ability to influence public debate once told me that all responsible journalists had a duty to denounce the fad diets that fill the press. They set people up for failure. Getting fit and losing weight was not one big thing but many little things. And the little thing that has given me the greatest pleasure has been running.

When I tried again this year, I learned from my previous mistakes. You do not just start running. You build up gradually. The smartphone was my saviour. You can download any number of “couch to 5k” programmes. A typical routine will have the instructor telling you to run repetitions of 60-second runs then 90-second walks in your first week. By your fourth, you will be running in five-minute bursts. By the fifth, you are managing eight-minute runs, and by the final eighth you should be able to cover three miles, 5km, as you run for a full 30 minutes. I never thought I could do it. Running for three minutes caused the shooting pains to return, but after eight weeks I managed 5km. Nearly everyone can. Again, I am well aware of the danger of sounding like an evangelical preacher or self-help booster. Of course, there are people who cannot run. The point is that everyone without a relevant disability should be able to do it.

Smartphones have a further attraction to the typically embarrassed British man or woman. You do not have to think about the absurd figure you cut exercising in public. You plug in your headphones, turn up the music and blot out the world and all its cutting glances.

Once you have covered 5km, you face a choice – just go on jogging or try to become a runner. Parkrun is there for you, if you choose the latter. It is one of those admirable organisations, which, along with Alcoholics Anonymous and Citizens Advice, represent the big society David Cameron never understood. Staffed by volunteers and entirely free, it organises runs for hundreds of thousands of people. You sign up online, go along and find you’re in a competitive race. Running is an egalitarian sport. Until you reach a high level, the only person you are competing against is yourself. Parkrun officials time every entrant’s run, and it was with great pride that I saw my 5km time fall from a sluggish 27 minutes to an almost passable 24.

Taking running further means intensive runs to increase your speed and long runs where you just go as far as you can for as long as you can. These to me are the real joy. I have reached 10 miles, and it is on these long runs without pressure or competition that I have experienced my moments of elation. They come early in the morning, when the sun is just coming out, and I find myself moving fast, and my feet are hitting the ground lightly. For someone who was fat and slow for decades I found my sense of geography changed. I am able to flit through gaps I would previously have squeezed through. My sense of space has changed too. Distances I could never think of travelling by foot are now manageable. Seven miles? I could cover that in an hour. Indeed I want to get out and run it now and feel almost bereft when a pulled muscle or work keeps me indoors.

In his beautiful memoir, What I Talk About When I Talk About Running, the Japanese novelist Haruki Murakami thinks about different types of creativity when he runs. Writers who are blessed with inborn talent find their work flows like water from a spring. Murakami and most of the rest of us don’t have a natural talent. “I have to pound the rock with a chisel and dig out a deep hole before I can locate a source of creativity,” he says. For me running is like that. It is hard and painful, a long pound. But every now and then the water springs out of the rock and I am carried along with the flow.

Murakami also warns against evangelising. You should not try to convince others to run, he insists. I have broken his rule. Not because I want to sermonise or fat shame you or shout at you like a brutal PE instructor, but because for this tired, slow, middle-aged Englishman running, however inadequately, is a liberation.