I was late filing my column this week – late because I've been trying to identify the faint sense of unease trailing me around all day, every day, and I think I've finally got it nailed. It's that I've got my seasons arse about face and I have lived my year backwards, leaving me with some interesting decisions to make for autumn and winter in terms of what to wear.

It started to go wrong in May, when the weather was dire and awful and it felt like winter again. I was spending a lot of time working at home. Then it picked up briefly over the Olympics, when we had what passed for summer this year, and now I'm back again to sundry items of nondescript, multi-seasonal casualwear. My summer clothes have barely seen the light of day, let alone a ray of sunshine.

As a not-entirely-unexpected side effect of spending much of the last four months sitting on my backside in front of a keyboard, or stretched out on the sofa with the cat cheering on Team GB, I find that, underneath the layers of jersey and loose shirts, I've developed, ahem, a spare tyre. And the spare tyre also has a spare tyre, one that has settled around my bum. I am, oh woe, a whole dress size bigger and my wardrobe choices are limited by what still fits. It will not do. It will not do at all.

I should make a point here: it's not entirely my fault. Yes, yes, there's my unfortunate and well-documented fondness for Ben & Jerry's but that's a treat I allow myself on Saturday evenings, and Saturday evenings only. Saturday Sweetie Night is a discipline I've carried on from when my children were still at home – no sweeties all week and then we'd consume our own body weight in Cadbury's on a Saturday night. The rest of the time, I assure you, I am the very soul of abstemiousness. I eat salad and fruit, yoghurt and muesli, and very few carbs. I don't drink a daily latte any more and my alcohol consumption has fallen since I started working at home. Gawd, I'm almost teetotal!

Nope, a good part of this is down to that age thing no one tells you about – a lowered metabolic rate. I remember my mum at the age I am now, in deep distress because she only had to look at a buttered crumpet to put on half-a-stone. There is a limit to how far I'm prepared to go over the appropriate BMI for my height and build, and not because I'm obsessive about slimness – I'm so very tired of seeing gaunt-faced social x-rays nibbling on an olive and then agonising over it – but there is a direct correlation between some health issues and where the digits stop reeling on my bathroom scales. As my dear old dad keeps saying: "If you haven't got your health, you haven't got anything."

So what's the plan? Not too much tinkering with my daily diet but rather an improved level of physical activity. When I was commuting every day I used the London transport system as my daily gym – walking up and down escalators, power-walking along the pavements, swearing at dawdling tourists who got in my way. When the Olympics began, my daily yomp around Greenwich Park was curtailed, but now the stadium and fences are coming down I'm working my way back up to a full five miles most mornings. I feel better for it and, honestly, I can already see a difference. Hurrah! I will soon be able to wear my favourite skirt again. I am, in short, giving my metabolic rate a kick up its very own backside. Now then, where did I put my special ice-cream spoon?

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