Ruth Walker: If I don't want to develop a derriere the size of my January gas bill, I need to be raising my activity level

OKAY, that's it. We're a week into 2010 and there can be no more excuses. The sofa has developed a recognisably Ruth-shaped dent in the cushions, the batteries have died in the remote and even the cat is turning its nose up at any decidedly whiffy leftovers.

The tree is blowing around the garden, naked and abandoned, waiting for the council to collect it, the cards are in the red recycling bin, the children are back at school and I'm back at work. I've taken my annual trip up to the attic (what is all that crap, anyway?) to stash the decs and the only reminder that I have just survived a fortnight of gluttony is the straining button on my skinny jeans.

Yes, festive sloth is fine and dandy, but all good things must come to an end. And if I don't want to develop a derriere the size of my January gas bill and the texture of bread sauce, I should really be doing something to raise my activity level to somewhere above the semi-comatose.

I'm not one for resolutions – I don't smoke so can't stop, I already eat pretty healthily (most of the time) and all the rest of my little weaknesses (designer footwear, expensive dark chocolate and cheap white wine) I wouldn't give up if my very life depended on it.

So that only leaves the getting fit option. I know, it's pretty predictable, and since the bike is hibernating in the garage until the nights draw out again I shall need to address alternative ways of incorporating exercise into my life.

To start with, I got a personal trainer in my stocking. It was a bit of a squeeze, admittedly, but we had our first session this week and I have high hopes for what we might achieve together. My only concern is that his name – Will Power (yes, really!) – doesn't turn out to be a poor stab at irony.

I also joined a gym. Well, that's not entirely accurate. I joined on behalf of The Mild One, in an attempt to curb his increasingly wild ways. I'm hoping that he might sign me in every now and again.

He had better, because in March I will be taking part in the Meadows Marathon. It's a student-run event, so if the exertions themselves don't do it, just being surrounded by fit young things half my age should succeed in making me feel appropriately decrepit.

"The Meadows Marathon is perhaps the most vibrant and exciting half marathon in the whole of Scotland," says John Owens, editor of myRace magazine. "The atmosphere on the day is electric and it's fantastic to see so many students and keen runners taking part in the name of charity. The costumes last year were well thought out, and it will be interesting to see what some runners come up with this year."

My costume will be that of an exhausted fortysomething on the verge of a coronary.

Meanwhile, eagle-eyed readers may have noticed that the title of the marathon is slightly misleading, as it is, in fact, only a half marathon. But, believe me, it's no less daunting for that.

I've also signed up two of my fitter friends for support so there will be no excuse for not keeping up with a training schedule of sorts.

But, hey, the Meadows is still two months away – there's plenty of time. All that talk of working out has made me quite puggled and I think I've just spotted a mince pie that's still on the right side of its sell-by date.

I'm off for a lie-down – the new regime kicks in tomorrow.

• This article was first published in Scotsman on Sunday on 10 January, 2010.