The current debate about the ever-growing pressure on social care services is by no means the first, and certainly won’t be the last. But it is always predicated around the same central assumption: with increasing numbers of older people, the costs can only go up. This is to miss an important point, one well known to public health experts but strangely ignored by most politicians. The problem isn’t just about an ageing population, it’s also about an increasingly sedentary one.

That far too many Britons are not physically active is well documented, even if the sheer extent can sometimes be surprising. For example, a fifth of Scottish people say they have not walked for more than 20 minutes even once over the past year. Not once. Also well known, if less discussed, is the toll this takes on the public health. The usual estimate is that about 85,000 people die early each year in England and Wales due to illnesses caused by sedentary living, mainly heart disease, type 2 diabetes and various cancers.

But callous as it sounds, what really keep senior civil servants awake at night isn’t the mortality rate; it’s the parallel issue known as morbidity: the quality of life people can expect as they age, and thus how independent they remain. Here, the near-magical effects of keeping up even a fairly minimal regime of exercise are also well documented.

Physical activity keeps people stronger and more supple as they age. It also improves balance, gives better bone density and makes them less likely to be depressed or develop Alzheimer’s. These are all things associated with needing less social care.

The earlier people start such a regime, the better the effect into older age. One public health expert once told me he encourages people this way: “I tell them, ‘being active throughout your life is about being able to get to the loo on time in your old age’. They can get their heads around that.”

If it’s so conclusive, why is the government not pushing this as part of the solution? Here we get into the more tricky area of what is politically acceptable, and whether ministers want to embark on what is, in effect, low-scale social engineering. Decades of experience has shown that just encouraging people to exercise has limited impact. Urging people to go to a gym or walk briskly round their local park eats into what is known to experts as “discretionary time” – when they could be doing something else. Thus they tend to give up pretty soon.

One of the few exercise regimes proved to stick is active travel, more specifically making walking or cycling to work, school or the shops sufficiently safe and convenient that it becomes easier for people to do it than not. Cycling is arguably the mode that should be encouraged the most. Aside from the fact people can cover greater distances on a bike than on foot, studies have shown that the slightly greater exertion it involves brings even more miraculous health benefits.

The mechanics of how you nudge people away from cars and on to bikes is another discussion, but there is little doubt it can be done if the political will is there. The Netherlands and Denmark, for example, have spent decades very deliberately re-shaping their road environments away from the car culture of the 1960s and 70s towards mass cycling. It’s not an accident.

Yes, it would take the UK a decade or so to make similar changes. But given the vast and unquestioned public health benefits on offer, with the huge savings this would bring on social care (not to mention the NHS, which some experts predict will soon become bankrupt due to the costs of ailments linked to sedentary lives), it’s a bit depressing that we haven’t even begun the debate.

For all the occasionally hysterical media narrative on cycling, it barely exists as an issue at the top of government. In his first interview as transport secretary last week, Chris Grayling spoke of cyclists only to complain about those who jumped red lights and to suggest that bike lanes caused problems for “road users”, a group that apparently doesn’t include people on bikes.

Grayling’s predecessor in the job, Philip Hammond, now the chancellor, was reported last month to have urged London’s mayor, Sadiq Khan, to rip out one of the few properly designed and safe bike lanes in the entire country. This is not to castigate the pair personally. Barring a handful of exceptions, almost all Britain’s top politicians think much the same about cycling as everyday transport.

The debate on social care shows how blinkered this is. As part of a book I’ve been writing about the beneficial effects of more cycling, I spoke to a number of public health experts about the problem of sedentary living. Several of them used the same analogy: getting people active in their everyday lives is, they said, like “a miracle pill” in the way it wards off disease and decrepitude. That miracle pill is well known. We’re just waiting for someone with the political vision to administer it.

• Bike Nation: How Cycling Can Save the World, by Peter Walker is published in April