My maternal grandfather, Percy Hazzard, would have liked to have been a career soldier, but he was a farmer. He drove his tractor, milked cows and reared pigs in Somerset – and took to rural pursuits, such as fox-hunting, with fervour.

When Percy died, he left the farm to his three daughters, who all ended up in London. I never knew him, but I felt a great affinity for him. From an early age I was besotted with ponies, then horses. It wasn't easy to get close to them, living in London, but I'd bunk off school to visit a livery stable in Hertfordshire and didn't care what trouble I got into.

My mother and her sisters were forced to sell the farm and for years blamed one another for the loss of something that none of them wanted. But I wanted it. At the farm there were horses. People there rode, jumped, raced and hunted.

I managed to get into an agricultural college in Oxfordshire. I rode horses first thing in the morning and last thing at night, and in between times I groomed, studied and cosseted them.

I did some writing for Pony magazine. I liked writing. I came back to London and landed a job working for Lew Grade in the press office at ATV. Then Fleet Street beckoned – long lunches and bonhomie. In tandem, I made a succession of documentaries – mainly about horses. Then I had a bad accident while hunting in Ireland and it shattered my nerve. I thought I'd never get back on a horse and for 20 years I didn't. It seemed my passion had gone.

I married Rose, the greatest cook in Britain and one of the great food writers. In our kitchen there is constant experimentation; there are new recipes to be tested, leftovers to be turned into second dishes. A Bolognese sauce whipped up from the rib of beef eaten the previous Sunday. Wild duck, pheasant, partridge and grouse. Pasta with thick, rich sauces. Hot curries and mild ones. The food is relentless and the quantities huge; the same goes for the wine.

My weight didn't pile on overnight, but when I went to buy new trousers I realised I needed to go up a notch or two, shirt collars got tighter, running for a bus became uncomfortable. The children would occasionally tease me, the paunch grew and grew. The scales creaked at more than 16½ stone.

Today, I celebrate my 50th birthday. Three years ago, almost to the day, I had a discussion with Rose. Bloated after the Christmas excess, it was one of those "So what are you going to do about it?" chats. I was touched by her concern: "Are you worried I might die?" I asked her.

"No," she responded. "I'm worried that you might have a stroke and I'll end up looking after you for the rest of my life. If you won't lose weight for me, what would you do it for?" And then the words came out of my mouth. I don't know why and can only guess that I had been meaning to say them for a very long time. "I'd lose weight to ride again."

Rose thought I meant hacking through woodlands. I could see she was unimpressed. Then I said: "I'll do it to ride in a race, on a proper racehorse, on one of Britain's race tracks." That stopped her in her stride.

For the next 10 months I put all my effort into pulling it off. I fasted, got fit, lost five stone and … entirely neglected my family. Gradually, with exercise and diet the weight dropped off. I would take my daughter to and from her primary school on my bicycle – three miles there and three miles back twice a day. I would stand on the pedals, imagining they were stirrups, the bike was my steed. Bill, our cross-eyed cocker spaniel, came with us.

I would decamp to our house in Dorset with Jack, then 13, and Lara, 11, to ride Daz, a giant one-eyed monster of a horse. At more than 18 hands, Daz was a thoroughbred crossed with a shire horse. As my confidence grew, I graduated to Edward, an ex-racehorse stabled in the village. He was faster, but I survived. We would go for long canters, amble through woodland, and trot for miles along roads and country lanes. I'd been warned that he was capable of bucking ferociously, but he never so much as raised a hind leg.

One day I met a parent at the children's school who said: "I hear you are going to be a famous jockey." It seemed that Lara had told her class just that.

Once I had lost three stone, I decided to go and live the life of a stable lad. My friend Charlie Egerton, a racehorse trainer with a better runners-to-winners ratio than nearly any other trainer in Britain, foolishly agreed to take me on. Rose likes Charlie, but she doesn't like horses. She thinks they are dangerous, stubborn and frightening. "Charlie will look after you won't he?" she asked.

Every Monday evening I would drive the 70 miles from London to Heads farm, just outside Lambourn in Berkshire. I would eat little and go to bed early. By 7am the following morning I would be on top of whatever horse I'd been allotted to ride. From tacking up to washing down and putting the horse away took an hour.

At 8am, I would ring the children to wish them well for their day at school. Lara would always ask about the horses, which one was I riding, how fast had we gone. Then Jack would come on and tell me what he was having for breakfast. My absences were tinged with sadness, but I knew if I was going to ride properly it would take dedication and discipline.

Riding a racehorse is a very different proposition from riding any other horse. You sit on a tiny patch of leather that passes for a saddle, and your stirrup leathers are pulled up very short. It's bloody dangerous. Jockeys and stable lads suffer horrific, crippling injuries and the mortality rate is high. Looking in the accident book on my first day, I was horrified by the catalogue of injuries – from missing teeth to broken legs, a detached patella and a fractured pelvis. And these were lads who knew what they were doing around horses. Last year the Injured Jockeys Fund provided financial assistance to more than 500 jockeys.

I was lucky. I didn't fall off, although I did have a minor infraction with my horse, Dancing Marabout, when he smacked me squarely in the face leaving a nasty gash below my right eye. The blood shot out. It looked terrible. Seeing my reflection in a window, I was shocked. I went into the house, took pictures of my battered face on my mobile phone and sent them back to the family in London.

Gradually, Rose and the children realised what I had embarked on. I asked them not to be too shocked when they saw my face – the injury looked a lot worse than it was. But Lara had an "Oh, my God!" moment. Rose offered some arnica to help bring down the swelling and Jack, a keen rugby player, concluded: "Horseracing is way more dangerous than rugby, Dad."

Writing, by which I had earned my living for 30 years, got left by the wayside. I spent much time staving off an irate bank manager as my overdraft rose. I juggled money as best I could, and Rose became the breadwinner. Meanwhile, when there were difficulties at home, all the responsibility for holding things together lay with her.

But the pull of the horse is so strong that you overcome virtually anything to be with them. Horses are an addiction. Professor David Nutt, the previous government's drugs adviser, identified a condition that he termed "equasy". It is a condition I have had since the age of five, when I sat on my first pony.

After riding, I would go running. Later, I would climb out of my evening bath dizzy and unsteady on my feet, then fall into bed for an exhausted sleep that would last for maybe two hours. Jockeys brutalise themselves like this every day. I did it only between Monday evenings and Friday afternoons. In the public baths at Wantage, I'd sweat off weight in the sauna, sometimes half a stone in a day. On trips back to London, people would stop me and ask if I was OK.

The Saturday before my race we had friends to dinner. The table groaned under the food, the children wolfed it down. Wine flowed for Rose and our friends. But I sat at the end of the table, having been alcohol-free for four months, chewing on a stick of celery.

I had been selfish. I had given up family life to transport myself into a different world: no more school runs, no supervising homework, no family dinners. But losing five stone in nine months was worth doing and, more than that, I'd fulfilled a dream and witnessed a life that might have been mine had things turned out differently.

Despite my four-month absence, when the big day came on 6 November 2008 at Towcester racecourse in Northampton, my family were all there: my mum, Rose, Jack and Lara. Jack came up to me and pushed a scrunched-up piece of paper into my hand. Written on it was: "I love you." Just before 4pm, 12 riders stormed off. Dancing Marabout settled in second place and down the hill we went, then round the right-hand bend at the bottom and along the flat for half a mile before a five-furlong climb to the finishing post.

I was still lying second, but AP McCoy, the greatest and most successful jockey ever had warned me: "The hill at Towcester is a bastard." And so it was. We ran out of puff and I came fifth.

It felt good. So good, indeed, that nine months later I did the entire thing again and raced at Newbury. In better conditions, on a better horse, my hopes were high … until I got on the scales. The diet this time had not been so successful and carrying an extra stone was my undoing. We finished seventh. But I hope Percy Hazzard would have been proud.

Jumbo to Jockey by Dominic Prince is published by Fourth Estate, £10.99. To order a copy for £10.49, with free UK p&p, go to theguardian.com/bookshop or call 0330 333 6847

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