Ruth Walker: I'd handled childbirth with less fuss – though, admittedly, that was with the help of some hard-core narcotics

FIRST, I have a small confession to make. At the time of last week's column going to press, I sincerely believed I would have survived my first session with Fit Guy, my new personal trainer, creator of the 1,000-calorie-an-hour workout and the man who is going to turn me from post-Christmas slob to marathon woman by March (stop laughing at the back).

But then the lurgy struck, forcing me to abandon all previous arrangements. Though, truth be told, a combination of this being the coldest winter in living memory and me being as poor as a church mouse meant my diary featured nothing more pressing than an appointment with the accountant to do a rush job on last year's tax return and a couple of tumbleweed blowing aimlessly across January.

But I digress. And since a hefty dose of Beechams and several boxes of Kleenex Balsam have seen off any lingering germs, I'm back to 100 per cent fitness and am happy to report that training has now commenced.

"What are your objectives?" asked Fit Guy cheerfully as I pedalled up a virtual Ben Nevis, learning, in the process, that polite conversation and cycling do not mix.

"I... puff... want a... gasp... flat stomach," I replied, somewhat breathlessly – though not, in my humble opinion, unreasonably. I may have delivered three not insubstantial human beings into this world, but surely with the right motivation and more crunches than a greengrocers' convention, a washboard middle might be achievable.

FG laughed. Not a little snigger, or even a barely concealed chortle, but an out-loud, no-holds-barred guffaw (a belly laugh, if you will). Once he had regained his composure, he added, "I'm so glad you gave me something easy to start off with."

I think he was being sarcastic. But I'm a forgiving sort, so we moved swiftly on to the weights, while the conversation moved on to more appropriate territory: strengths and weaknesses. At school I was more of a sprinter than a long-distance runner, so I perform better on short, fast bursts and not so well on anything requiring stamina. Which might explain my poor track record when it comes to relationships – and rather puts the planned March marathon in jeopardy.

Never mind. FG likes a challenge, and everything was going swimmingly until, around 600 calories in, I felt a little faint. And sick. Which is how I came to be lying flat on my back in the middle of the gym, groaning (or maybe that was FG – it was hard to tell with a wet towel over my face). What was wrong with me? I'd handled childbirth with less fuss – though, admittedly, I'd managed that with the help of some very hard-core narcotics.

And here comes my second confession: he's not the first trainer to make me ill. Once, mid-kettle-bell workout (they're the kind of weights Russian shot-putters train with), I had to sit with my head between my legs and gulp down a large glass of orange juice to prevent me from passing out, while the big Aussie lump looked on with a glint of victory in his eye.

For days afterwards, my thighs were so stiff I walked with a limp and had to go to the toilet standing up.

"How much have you had to eat today?" asked FG suspiciously.

A flashback to lunch revealed me lingering over the crispy-topped macaroni cheese on the menu, only to have an uncharacteristic twinge of conscience and opt for the lentil soup instead, accompanied by two dry Ryvita. No wonder my body was in a state of shock.

So, from this week, the diet has been altered to include a pre-workout snack and a (gasp!) post-lunch carb ban. That means, of course, that if I want to achieve that fantasy six-pack I'll have to consume my entire chocolate and wine intake at breakfast.

It's a tough job, but I think I'm up to it.

• This article was first published in Scotland on Sunday on 24 January, 2010