Adrian Chiles: New year is a time for kicking bad habits - take spuds, for instance

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To motivate yourself to take more exercise and improve your diet you're sometimes told to stand naked in front of a mirror. I don't think this works.

To motivate yourself to take more exercise and improve your diet you're sometimes told to stand naked in front of a mirror. I don't think this works because however overweight you are, by the time you're as old as me (37 and three-quarters), you will have found a way to pull your stomach, chins or whatever into a position that flatters you in your own eyes. You'll have seen yourself in the mirror so many times that no matter how out of shape you are, you will no longer be able to shock yourself.

Much more shocking in my experience is a photograph, preferably one taken from an unfamiliar angle when you're not posing for it. For me, the straw that broke the camel's back was a photo I was actually posing for. It's the photo which adorns this page. It was taken for the BBC's publicity department. When I first saw it about this time last year, I knew that something had to change: touching 17 stone I really was too fat.

Thousands of you will be reaching a similar conclusion about yourselves today. And you'll probably join the queue to join a gym. Financially, this is precisely the wrong time of year to be joining a gym. Show up in July and they'll bite your pudgy hands off, but just now everyone's had the same idea about themselves so they can charge what they like.

If you take no exercise, then taking some is obviously the answer. The trouble I had, this time last year, was that I was already doing loads of exercise - running, rowing, swimming or cycling at least every other day for at least 40 minutes, and often for more than an hour. I felt fairly fit but I remained fairly fat. On one hand there was no mystery in this as I was always eating and drinking fairly prodigious quantities of everything. On the other hand I was doing so much exercise I reasoned that I needed the fuel inside me. The truth was that I was running to eat and eating to run.

It had always been like this. Ten years earlier I tried to tackle my weight problem by cycling to Zagreb. I got the boat to Hamburg from where I pedalled all the way to the capital of Croatia via Berlin, Prague, Vienna, Bratislava and Budapest. Every day I cycled to exhaustion; every evening I ate myself to exhaustion. I was sure I was losing weight, though it was difficult to be certain when all I ever wore was cycling shorts or tracksuits, both of which obviously lacked tell-tale belt holes.

On the very last day, within sight of Zagreb, I passed an infants' school. It was playtime. A number of the apple-cheeked little blighters pointed at me and laughed, making no attempt to hide their amused scorn. One of them shouted: "Eh bucko, pozuri se!" Translation: "Hey fatty! Get a move on!"

Between Hamburg and Zagreb no mountain had beaten me. But that little urchin left me bowed and broken. I limped into Zagreb proud to have got there, but disconsolate that I obviously hadn't turned myself into the lantern-jawed, wiry transcontinental cyclist chap I assumed I'd be.

I spent the next 10 years running my legs off. Every footpath in west London has my footprints on it and wherever I went travelling my running clobber went too. I ran the New York marathon, narrowly breaking the psychologically important five-hour barrier. In the London marathon I narrowly (OK, by 18 minutes) failed to break the four-hour barrier. So many hours spent out there pounding the streets; so many arguments with my wife over the sweaty pants she's always picking, gagging, out of the dirty-washing basket; so much time and effort. Weight loss: zero. Whatever I did; whichever training regime I adhered to, my weight never dropped below 16 and three-quarters. The only compliment I ever got on my physique was from people in the street who'd stop me to tell me I wasn't as fat as I looked on the telly.

There were other signs that I was making no progress: the one which killed me the most was being overtaken when I was out running. I never minded being overtaken by a proper runner: you know, one of those skinny, bouncing skeletons who runs for fun like you might run for your life.

Fine, I'll never be like them. But God spare me from being passed by people running slower than me. It's a phenomenon never covered in my O-level physics course but I swear it's happened and it breaks my heart. There I'd be on the Thames Path, pretending to be Steve Cram, and someone older and fatter than me in knackered, 25-year-old Green Flash pumps would lollop - which is not even a jog - slowly past, feigning nonchalance.

I just couldn't take it, so I'd only run on popular runners' routes like the Thames at ludicrous times of day and/or in terrible weather. In this way, I could hope to lollop alone. For years I never ran along the Thames in nice weather at the weekend - the overtaking would make me homicidal. If there's run rage like there's road rage, I'm the one who invented it.

Then, on this day last year, everything changed. I made the only new year resolution I've ever kept: I gave up potatoes, pasta, rice and bread. I'm not quite sure why I decided to do this but the effect has been dramatic. It's achieved what running nearly 2,000 miles a year never achieved: I have lost two stone in weight; three notches off my belt; two inches off my collar size; four inches off my waist.

Don't get me wrong, I'm no featherweight, but I do now occasionally actually overtake people. It feels good, though my resolution this year is to stop being smug about it. Even now you might look at me and quite reasonably ask if advice from me on weight loss has any more value than Wayne Rooney's advice on anger management.

But for what it's worth, here it is: never mind the gym - just calculate what you'd spend on potatoes, rice, pasta or bread this year, and spend it on decent running shoes instead. Hit the streets. Just don't overtake me unless you're going really, really fast.