LAST WEEK came what passes for good news in the fight against fat. Over the last decade, the government reported, Americans have not become more overweight. Of course it’s also the case that, despite great effort, we are not getting any thinner.

But we have been thinking about the whole issue the wrong way. If we truly want to become healthier, then the first step is to not think about our weight.

This would seem to defy all logic – we solve our weight problem by ignoring it?

Evidence suggests that the most important thing we can do for ourselves is not to shed pounds, but to achieve the most basic level of fitness. And it doesn’t take much to be fit. Put aside visions of gym memberships and a relentless personal trainer. All it takes is a modest amount of regular walking – say 30 minutes, five times a week – to reap dramatic benefits for your mental and physical health. You don’t need intensive cross-training, you just need to move.

This suggests that our idea of fitness needs a fundamental rethink. When we speak of “getting in shape,” we think “losing weight.” We tend to set unrealistic weight loss goals and then, a few weeks after a solemn New Year’s resolution – say, around now – we start to go rogue. We’d all be a lot better off if, instead of asking “how many pounds did I lose this week,” we asked, “did I go for a stroll today?”

“We need to change the game a bit,” says Dr. Mike Evans, a staff physician at St. Michael’s Hospital in Toronto and an associate professor at the University of Toronto. Evans is a straight-talking family physician who’s the brains behind a new animated YouTube video that argues for a change in attitude. It’s going viral, and it is my hope that every man, woman, and child in the country watches it.

Some of the evidence he cites: A study of thousands of Harvard alums found that the fitter ones had a 23 percent lower death rate. Moderate exercise relieves depression by 30 percent. It reduces anxiety, slows the progression to Alzheimer’s and dementia, relieves knee arthritis, and on and on.

In fact, when Evans looks at his patients, he sees “sitting disease.” When we are not asleep, we are sitting – in a desk chair, in a car, on a couch, remote in hand. Compared to humans of a thousand years ago, we are grossly overweight, yes, but we are also shockingly inactive.

Work by Steven Blair of the University of South Carolina has shown that improving fitness can counteract many of the risks of obesity. Losing weight will help, yes. But, as Blair told me: “Low fitness is far more important than fatness as a predictor of mortality.”

And the good news is that people get the largest benefit when they shift from nothing to something – from utter inactivity to almost anything. Honestly, your personal trainer could be a dog. A recent study found that nearly half of dog walkers exercised 30 minutes a day, at least five days a week, compared to just a third without dogs. Another study of people living in an assisted living facility found that giving older adults a canine walking partner increased fitness more than pairing them with a person.

Dogs don’t appeal? Take a walk after lunch. Or ride a bike. It’s not that hard.

None of this is to say that the country doesn’t need aggressive efforts to improve our diets and boost exercise. Last year the Boston Foundation and NEHI, a health policy institute, published a report card which gave Massachusetts mixed grades – two Bs and a D – on encouraging physical activity.

Nor should this be an excuse to pass on other steps to improve your health. Quitting cigarettes, drinking in moderation, controlling blood pressure and cholesterol – all these are basic things you should be doing, along with smarter choices about food.

But if you are going to do only one thing, then you should take up the challenge Evans puts so brilliantly in his video: Can you limit your sitting and sleeping to just 23.5 hours a day?