Last month, my only regret was the dearth of mince pies. Being away for half of December meant just one specimen got past the drawbridge before I headed off to the land of panettone and cantucci. Thankfully it was perfect: rich, fruity, laced with brandy, and encased in pastry so light it tap-danced on its way down.

When we returned, my shopping trolley veered towards shelves stacked with mincemeat, but unfortunately the Minister for the Interior was on duty. This invisible rottweiler, gatekeeper for every calorie that hoves into sight, was unforgiving. I blame the news that four out of five of us between 40 and 60 is obese, or drinks too much, or is unfit, or all three at once. Reluctantly, I fixed my gaze on the salad counter and passed by.

Who would have thought that a mince pie could be a matter of national significance? When Winnie the Pooh disappeared with a large pot of honey, or Bridget Jones confessed to gorging on Milk Tray – “best to get rid of all Christmas confectionery in one go and make a fresh start tomorrow” – we assumed that was entirely their own business. If someone wants to stuff themselves with sugar or fat, who are we to sneer or criticise?

Yet in light of the alarming level of obesity and unfitness the middle-aged embody, it is clear that the personal is indeed becoming very political. Those of us who pile on the pounds and take no care of our welfare are storing up trouble for the years ahead. Until recently, however, that decision was seen as the individual’s right. In the same way that smoking, or drinking to excess is a choice, overeating or barely moving beyond the couch is an exercise of freewill. Doctors can do little more than offer counsel. To intervene, except in extreme cases, would be a breach of the natural law that our life is our own to do with as we like.

But that looks set to change. And, given the expanding girth of the average citizen, perhaps it is no bad thing. Now to the usual warnings of what excessive weight and alcohol and inertia can lead to – heart and liver disease, strokes and diabetes head the anaconda list – is added the unpalatable truth that if this age group does not mend its ways, it will cost the health service billions.

So no longer is that mince pie a matter entirely between your conscience and commonsense. It assumes massive proportions when health professionals talk in terms of avoidable spending and unsustainable strain on budgets. Almost overnight it seems the tone of the message has changed: we are encouraged to take action not merely to improve our health but also, and equally importantly, to ease demand on the public purse. It is a chilling new outlook, even if justifiably so, and one that only the most pig-headed or those who lack willpower can ignore.

Clearly what most of us needed this Christmas was not an i-pad or Nespresso machine but a pair of scales. A friend bought a new set last week, determined to take action. Her husband stepped onto them for the first time in a decade, and was quietly pleased. Wisely, he did not ask his spouse what she weighed. I suspect most of us would rather discuss our sex lives or salaries than reveal our poundage. As one who only recently has begun weighing myself regularly, I know the dread of watching the quivering display, and praying it won’t rise any higher. When I first approach the machine, I give it a gentle prod to waken it up, in the hope it will be in a good mood.

It is not a cheering way to start the day. Increasingly, though, I’m coming to think it is necessary, for all of us. Whether we’ve only a flat tyre around our waist, have stones to shed, or simply want to stick with the status quo, wishful thinking or promises about beginning a new regime next week merely postpone the discomfort or chagrin of facing up to reality. As the epidemic of poor health makes plain, it is our responsibility and duty to take our health into our own hands, rather than expect the NHS always to make us better.

That said, slimming is not easy. Reasons for overeating can be psychologically complex and distressing, and for the seriously overweight food is never just fodder or fuel. It comes freighted with issues that have nothing to do with nutrition or hunger, and which those of us with a normal relationship with food cannot begin to imagine. Overcoming the causes of obesity is as much about changing outlook and defeating demons as shrinking portions. Yet it can be done. Listening to those who have changed shape is deeply moving, as they describe the vastly improved life and self-confidence they enjoy.

Almost as hard as eating less is finding time to be more active. This pressure can seem like an intolerable extra burden on already over-timetabled lives. You can blame cars and sedentary jobs and being always busy for the fact that we are much less fit than our grandparents. Apportioning blame, sadly, does not change anything. In the past half century society has created a way of living that for most of us is harmful. Individually, we cannot change the infrastructure of our towns and cities, or the nature of our occupations, unless we choose another profession. All we can do is make small, incremental changes, and hope their benefits accrue.

Getting fitter is made immeasurably more difficult by the plethora of conflicting advice. From faddy diets – the 5:2, Atkins, Raw Food, Paleo and their awful like – to exercise regimes intended to tear muscles or turn wrinklies into mountain goats, it seems everyone has a tried and tested fix. The problem is knowing who to believe.

I read recently about a doctor who claims that eating breakfast, far from fending off hunger pangs, raises blood sugar levels and cravings. Better to fast until lunchtime, he advised. New research also concludes that dieters with realistic goals do not fare as well as those who set a scarily high bar, whose weight loss is far more dramatic.

I’m not convinced. The latest discovery is that our 1950s counterparts consumed less healthy food than we do, and in greater quantities, yet the average bloke was a stone lighter. Surely being more active is half the answer. Combined with a sensible reduction in food, a modest but sustained boost in daily exercise, such as brisk walking or vigorous housework, could work wonders.

Despite the seasonal avalanche of statistics and tips intended to spur us into new habits, getting fitter need not require fell-running or going to a gym, nor following a complicated diet. In fact, I’ve yet to hear better advice than in A Far Cry from Kensington, where Muriel Spark’s enormous heroine sheds her bulk by eating and drinking precisely half of what’s on her plate and in her glass. Spark, whose own weight often fluctuated, offered this regimen with no thought of NHS coffers. Like many solutions, however, the simplest can also be the best.