LONGEVITY is beginning to look like the race that can't be won.

Freed from the curse of early death, we start to realise that health, long life and happiness do not necessarily go together. Nor do they come cheap.

The headline statistic supplied by Macmillan Cancer Support is startling in any number of ways to the lay people among us. As a society we are living longer. This means, surely, that we are healthier. Yet by 2020, 47% of us can expect to have endured one or other form of cancer.

That's not what is commonly described as healthy. Equally, on one, semi-intuitive understanding of the situation, things are getting worse. In 1992, 32% could expect to suffer cancer at some time; in 2010, the figure was 44%. So we live longer because we are healthier, yet suffer more illness because we live longer.

I'm being simplistic. It is striking, though, that so many of the advances made in cancer treatment, so many of the necessary gains in health campaigns, especially against smoking, are being offset by the inevitable perils of longevity. For all the steps forward, there are steps back.

The Macmillan organisation is clear enough about the advances. As the chances of contracting cancer increase, the risks of dying from one of more than 200 types of the disease are falling. But as anyone who has been around a victim knows, survival and good health are not one and the same. Even when life is spared, cancer takes a punishing toll. If 47% is an accurate estimate, profound social consequences will follow.

The challenge of an ageing society can be measured in odd ways. A lot of us have heard, for most of our lives, that smoking is one of the great evils. As a cause of lung cancer, its effects are catastrophic. Slowly but surely, despite the best efforts of the tobacco industry, we (most of us) have taken heed. As health campaigns go, the long battle, if not yet won, has been a big success.

In 1948, fully 82% of British men and 41% of women were smokers. By 2011, the equivalent figures were 21% and 19%. In 1979, 45% of the entire adult population smoked; in 2010, the number stood at 21%. Yet despite it all, the chances of suffering from one of the group of diseases we call cancer continues to rise.

Crude figures hide a lot of complications. In 1948, there were millions of smokers, but no junk food culture. Obesity was all but non-existent. Alcohol abuse was less of an issue. Exposure to pollutants, like the flame retardants that turn up these days in the blood of infants, was not a conspicuous threat. Cancer campaigners worry about each of these.

The diseases of age were once rare for obvious reasons. Nowadays, most people expect to live beyond the familiar three score and 10. They hope to be maintained in decent health to the kind of age that would have been unthinkable to their parents, far less their grandparents. But as Macmillan Cancer Support is trying to make clear, the word debilitating does not begin to describes the effects of the disease on those who survive.

So who pays? How do they pay? How many trained people does it take to care for the numbers implied by that 47% statistic? Ciaran Devane, Macmillan's chief executive, was not kidding when he said that the prediction amounts to "a Herculean challenge for the NHS and for society". In his belief, the service will be unable to cope "without a fundamental shift towards proper after-care, without more care delivered in the community, and without engaging cancer patients on their own health".

He didn't put a figure on any of that. I'm not sure that anyone could come up with a trustworthy number. Yet we know, already, that governments of every stripe in every country are worried over future pension costs, future social security bills, and future strains on health services. We also know, from America's experiences, that turning health care over to private insurers is not a useful answer. Millions are abandoned.

The Macmillan's tidings arrive just as the biggest Westminster parties decide to agree that austerity will be with us for years, and probably until 2020. Fans of deficit mania and cornershop economics might tell you blithely that this is unavoidable, that social provisions are "unaffordable". That kind of happy talk doesn't solve a government's problem. The figure, you'll remember, is 47%. That's an awful lot of voters.

Those who are aged will be liable to recall that they have made plenty of contributions for many years. Some still want to believe they paid a lot of national insurance precisely to provide for their needs in old age. Yet the kinds of costs implied by the Macmillan predictions are big enough to make any government blink. The excuses have to grow pretty fancy if you are trying to fob off millions struggling to cope with the aftermath of chemotherapy.

A few things can be done before this society has to decide how it wishes to be ordered. With luck, what has worked partially with smoking can be made to work again. At the end of 2011, for example, Cancer Research UK reported that around 40% of all cancers count as avoidable. Smoking remains a big killer, but it would make a vast difference if more of us drank less and thought twice about what we eat.

Given that only between 5% and 10% of cancers are hereditary, the claim that a further 40% are avoidable probably offers the best hope for governments if they want to avoid the kind of situation foreseen by Mr Devane. Health campaigns and public education therefore become something more than a gesture towards common sense. They begin to sound like a national priority.

Equally, while obesity becomes a rising factor as smoking slowly diminishes, it is surely time for governments to begin to take the same attitude towards food as they do towards tobacco. It is illegal to sell cigarettes and booze to children, but you can stuff as much processed, addictive junk into them as you like. A society that defends the poisoning of the young in the name of "choice" is, at best, confused.

It's more complicated, of course, than calling for a ban on this or a ban on that. Those who manipulate the process of supply and demand are subtle. Behind the problem of junk food there are issues of poverty as well as addiction, of market dominance, advertising and political influence. Prohibition, in any case, has a poor record.

But if you pause to worry over the number of long-lived pensioners a working population can support, stop to think about the task when many are left frail, in pain, and in need of daily support. When all those "unavoidable choices" are already causing worries over home helps, district nurses, therapists and many other services, we need to think about what we want and how it is to be paid for.

If Westminster's politicians manage to destroy the concept of universal benefits by 2020, meanwhile, at least 47% of the population will be wondering about who they voted for, and why.