A study found those who did no previous exercise and those who were already fit both saw dramatic cuts to their risk of dying early when they upped their activity.
And the benefits were felt by people with existing medical conditions such as heart disease or cancer as well as those in general good health.
The researchers said their results showed it was never too late to get fit.
The study tracked nearly 15 000 Britons aged 40 to 79 over eight years to gauge how their exercise levels changed. They were then monitored over the next 13 years to establish how this impacted on their health.
Those who started exercise over the eight-year period having done none previously saw their risk of dying over the next 13 years fall by 24% .
Among those who were already active but increased their exercise levels, the risk went down by an even bigger 42%.
The University of Cambridge researchers said their findings offered hope even to those who have been inactive for years.
Writing in the British Medical Journal, they said: “These results are encouraging, not least for middle-aged and older adults with existing cardiovascular disease and cancer, who can still gain substantial longevity benefits by becoming more active, lending further support to the broad public health benefits of physical activity.”
Worryingly, however, the researchers said an increase in exercise levels was the exception rather than the norm and on average participants reduced their activity by 17% during the initial study period.
Health officials are increasingly concerned about rising levels of inactivity, with nearly half of adults failing to manage even a brisk 10-minute walk once a month. The NHS advises that people get 150 minutes of moderate exercise a week, such as cycling, swimming or gardening.
But the population, generally, is 20% less active than in the 1960s and on average walk 24km less a year than two decades ago.
We’re often told that our couch potato lifestyles are leading us to an early grave. But according to scientists, not all types of sitting are equally as bad for you. A study found that, while slumping on the sofa in front of the telly led to health problems, sitting at work did not have the same effect.
US researchers looked at the habits of 3 500 people and compared it with their health. They found those who watched more than four hours of TV a day had a 50% higher risk of heart disease and early death. But there was no difference between those who sat longest and sat least at work.
Happily, they found that moderate exercise can eliminate the harmful effects of slouching around.
Dr Keith Diaz, of Columbia University, said: “Taking a short break from the TV to go for a walk may be enough.”Daily Mail