There are moments in life when you feel the universe is telling you - as politely as possible - that you have become a Fat Bastard.
For me, the most crucial of those celestial hints came on 23 December last year. I was jabbering on my phone and hurried into my local KFC to inhale a mixture of lard, salts and chicken corpse when one of the staff exclaimed: "Johann! We have something for you!" And from below the counter, he pulled out a large Christmas card, signed by everybody who worked there.
"You are our best customer!" he exclaimed, and - in unison - the staff applauded me.
I half-expected Colonel Sanders himself to descend from the back room and smother me with his secret blend of herbs and spices.
This was not an isolated incident. Shortly before, I was watching television late at night, ambling through the channels pointlessly, when I burst out laughing.
I had stumbled across a person who looked like a really fat version of me. Chuckling, I texted a friend of mine who is also usually awake at 3am - and then suddenly it hit me. It was a repeat of a programme I had recorded a week before. It was no lookalike. It was me.
Oh, and when I interviewed the Dalai Lama, even he called me fat. When a man revered as an infinitely forgiving living deity calls you a munter, you take the hint.
Yes, I always prided myself on not caring about my appearance. I dismissed it as shallow blather, sucking time and energy away from doing things that matter, or just having fun. I saw exercise as dead time and food as fuel, to be inhaled as quickly and deliciously as possible.
Cooking was for wimps: if it hadn't been fried by minimum-wage workers in less than three minutes, I wasn't interested. But at the back of my mind there were anxieties I was poking away with a chicken drumstick or 10.
My maternal grandfather died in his early forties of a massive heart attack, and the single biggest indicator of heart disease is being overweight, especially around your middle.
I weighed 14 stone (88.9kg) and was 30 per cent body fat. If I were a sandwich, nobody would eat me except me. It wasn't deadly. I wasn't huge. But it was bad - I fell just within the technical definition of obesity - and I was edging upwards every year.
My idea of a balanced diet had been to eat McDonald's for lunch and Chicken Cottage for dinner. Indeed, I am a connoisseur of the chicken shops of east London - I can tell you the difference between Tennessee Fried Chicken, Best Chicken and Chicken Cottage (logo: a chicken grinning as it eats a chicken leg, because cannibalism is a great draw for the punters). But now my fried chickens were coming home to roost.
And so, at the start of my thirties, after being cheered on by the massed ranks of Kentucky Fried Chicken, I realised I had at least to try to change.
I had, in the past, had the odd temporary spasm, where I went to a gym for a few days - but I always tried to do too much too fast, injured myself within a day or two, and became convinced it was all impossible and I must never, ever go back. So what could I do differently?
I called the only four people I have ever known who had gone from being fat to being thin. One had a gastric band - which seemed a bit extreme for me. One contracted dysentery and several intestinal worms after falling into a sewage pit in rural India. I took notes of the location as a back-up.
The other two - both fellow journalists - got personal trainers. My immediate reaction was to think of my mother. She grew up in the Scottish tenements in a damp, rough neighbourhood, and she already thinks I am comically posh for writing in a newspaper and using words such as "expedite" and "imminent".
When she wants to impersonate me, she adopts the accent of the Queen and says: "Yah yah yah yah yah yah yah." I knew that if she heard I was paying somebody to make me exercise she would cackle with horrified glee.
"Can ye no go fae a run on yer own, ye fat bastard?" she would cackle.
"Wha' next? Will ye pay somebody tae wipe yer arse? Yah yah yah!"
I resolved to keep it secret from her.
I was told the best personal trainers in London are with Matt Roberts, who has offices across the city, in Hampstead, the City, Chelsea and Mayfair.
I heard rumours about the celebrities who go there - names I'm sworn to keep secret - and it was only after I had been going there for three months that I discovered in the newspapers that Matt Roberts himself has the tricky job of stopping David Cameron podging out, what with all the poor people whose flesh he is so cheerfully consuming.
Before my first session, I felt self-conscious and ridiculous as I trudged up the hill from Hampstead Tube. Why did I feel like this? What was going on? Why was I so anxious? At the back of my mind was the awareness that nobody had seen me exercise since I was at school. On the few occasions when I'd had to run in the years since - to catch a bus, say - I'd hated anybody seeing me breathless and unhealthy. Wasn't this just going to be a rerun?
I was greeted by Anna Milani, who is in charge of the Hampstead branch, and was to be my trainer. She looks like the physical incarnation of good health - a small, lithe former dancer from Italy who seemed to be made of vitamins, sunshine and protein.
I had to keep a diary of everything I'd eaten, and all the exercise I hadn't done, over the past week. We sat in a room downstairs and looked over it.
After she read the food list, she looked sympathetic and said: "This is a lot of fried chicken." And then: "Well, I admire your honesty."
She explained that if I wanted to become healthy, there was no short cut - I had to eat less crap, and exercise more. The most important kind of exercise was "cardio" - getting my heartbeat up so I would burn more calories. She was going to help me to do both.
Each session would last an hour, and I would come twice a week for six weeks - and then every weekday for another six weeks.
I changed into my "gym kit" - even the words sounded absurd to me - and we started on the treadmill. I was embarrassingly pleased there was nobody else there except me.
"To start with, we will just make you walk quickly, to get your heartbeat up," she said.
I did what I always do when I'm nervous. I talked. Incessantly. Even as I became breathless, I grilled Anna on everything about herself.
The Matt Roberts technique is to do lots of short bursts of intense exercise, so we skipped from one seemingly impossible exercise to another quickly.
One moment I was doing "walking lunges" - holding a dumb-bell across my shoulders and lunging forward across the gym, moving forward in long lunges of my legs - to sit-ups to lifting weights via weird contraptions I never entirely understood.
Each time, I did it wrong at first - holding it the wrong way, moving in the wrong way - and Anna gently directed me to do it in a way that would not dislocate my face (or something: I never really listened to the scientific explanation that followed "Don't do that, Johann").
For the first 20 minutes, I was suddenly struck by something. This was easy! It was nothing! Yes, I was sweating like the Niagara Falls. Yes, I looked like James Corden having an epileptic fit. But it was fine! I could do it!
And then it happened. As I was on the floor doing sit-ups again, I suddenly felt dizzy, and a crushing pain all over my body.
"Anna," I wheezed.
"You must ring an ambulance. At once. I have had a stroke."
She said softly, "You'll be fine, you just need a rest."
"No!" I howled. "This is like the start to an episode of Casualty! Can't you see I'm DYING?"
After sitting down for five minutes, glugging water and silently wailing, we started again. This time, I was slower. And dizzier. And more like a stroke victim. But I pressed on.
Anna was always the right balance of being encouraging while also being impatient with outbursts of nihilistic despair. Once the session was over, I was almost unable to walk down the stairs: my legs were so unused to that kind of exercise, it was like they had gone on strike.
And so it began. Twice a week, I would turn up, and feel like I had been disembowelled. At least, that's how it felt for the first 50 days. My legs ached. And when I say they ached, I mean they felt like they had been beaten with metal poles until they were broken. I discovered all kinds of exercise I never knew existed.
There is a Roman chair, in which you stand, support yourself with your arms, and then lift your legs up to exercise your abdominal muscles.
There is a machine where you stand against the wall and hold your arms up, as if you are about to be crucified, and then pull the weights towards your chest. And on, and on.
I felt pains in places that I had never thought of beneath the flab. At the start of every session, Anna would ask me very gently what I had eaten since we last saw each other.
When I told her, she would say very gently, "Oh dear." She suggested alternatives. I didn't listen to them.
But then, after about a month, something odd happened. I realised I was beginning to look forward to my sessions. It was getting less painful, and I was starting to feel a real head-buzz afterwards - a rush of happiness. Of course, I had heard of post-exercise endorphins, but I had assumed they were a mere propaganda-ploy to trick us into exercise. But no - here they were. And I liked them. It was all getting easier. And there were side benefits I hadn't anticipated. I was sleeping better. The small, nagging backache I'd had for the past year vanished. I felt more clear-headed.
And it was at that point that Anna's message about my diet began finally to percolate. After exercise, I didn't feel like going and dosing up on salts and fats. It felt less appetising. I don't know if it's that I felt I was undermining my own hard work by eating it, or that a healthier body doesn't need sudden bursts of energy from lousy external sources, but something was changing.
I listened to Anna and only ate low-fat proteins at night - boiled ham replaced fried chicken as a face-stuffer - and started gobbling fruit instead of fries. I even discovered I liked it.
And with each week, my body fat was dropping quite dramatically. After two months, it was down from 30 per cent to 26 per cent - which on the chart is down from "obese", through "very fat", to merely "fat".
I am merely fat! I cheered.
Before long, I was going every day. I was learning a whole new vocabulary. Before I went there, I thought hamstrings were a delicious snack, like pork scratchings. It turns out they are an essential part of your legs. At the end of every session, I was taught to stretch them, along with other parts of my body, so they didn't stiffen up and become painful.
Soon, I was going every day. I was - gasp! - enjoying it. It seemed surreal: to pack a gym kit and look forward to running or sweating or gasping.
Slightly surreally, my sessions were punctuated by the sight of celebrities - random celebrities, from television or sports - suddenly appearing next to me, equally covered in sweat, looking a little anxious and sheepish.
I know I wouldn't have motivated myself, on my own, to keep going - and I wouldn't have known what to do. I would have injured myself, or simply stopped.
After another month of going every weekday, my body fat was down to 22.7 per cent - which is in the middle of average. I weighed 12 stone (76.2kg).
People kept telling me I looked younger, healthier, happier... I can't stand looking at myself... but there seemed to be a consensus forming that I looked better, and I beamed.
And then, suddenly, I felt angry. It occurred to me that what I had been given so brilliantly at Matt Roberts was a physical education.
I had been taught how my body works, what will keep it in good condition, and what best fuels it. I had been taught how to exercise and stretch and eat. And I thought - why was I never taught this at school?
Yes, there is a subject called physical education - but it does precisely the opposite. Just a few phrases will remind every mildly unhealthy person in Britain of what that experience is like: "All four corners of the gym - go!" "Pick a team!" "Jump OVER the horse!"
Instead of teaching the fat kids who need it how to be healthy, PE teaches them that exercise is a humiliation and a source of shame to be avoided for the rest of your life.
Imagine if we taught reading by forcing the least literate kids to stammer and stutter over a book in front of the entire class, while everybody jeered at them.
Imagine if we taught maths by getting the kids who were good at it to pick teams and leave the least numerate kids to the end, with everybody crying, "We don't want that thick bastard!"
PE works very well for the kids who already enjoy running around healthily, and so don't need it, and very badly for the fat and unhealthy kids who desperately do need to learn about health.
Since school, I had carried a deep and profound sense that exercise was horrendous. And it isn't. It isn't at all. It's actually quite fun. But I had to be deprogrammed to see it.
Yes, I know I bear personal responsibility for it, too - nobody forced me to eat chicken popcorn - but I do think PE had a perverse effect on me and a lot of overweight people.
Now, I go for a run every morning - words I never thought I'd ever say, and that would cause my old PE teachers to splutter in disbelief.
But I will keep on my wall my Christmas card from the staff of KFC, as a reminder of where I would be heading if I let this sudden, exciting burst of exercise lapse.
"Happy Christmas Mr Johann!" it says.
"You buy more fried chicken than ANYONE ELSE and we LOVE YOU!"