A brisk 20 minute walk a day slashes the risk of developing heart disease in middle age couch potatoes by nearly a quarter, new research revealed.
Getting off the sofa and going for a walk or a bike ride for the recommended two-and-a-half hours a week time can prevent the chronic illness in just six years.
But failing to do any exercise or stopping can bring on the devastating order in people in their 50s and 60s just as quick.
And for those who did a bit of exercise but up their physical activity to the recommended time cut the risk by nearly a third.
Never too late to start exercising
The researchers from Johns Hopkins University in Baltimore, USA, said the findings suggested it may never be too late to reduce the risk with moderate exercise.
Senior author Professor Chiadi Ndumele said: "In everyday terms our findings suggest consistently participating in the recommended 150 minutes of moderate to vigorous activity each week - such as brisk walking or biking - in middle age may be enough to reduce your heart failure risk by 31 per cent.
"Additionally, going from no exercise to recommended activity levels over six years in middle age may reduce heart failure risk by 23 per cent."
It follows a survey by Public Health England last year that found 41 per cent of 40 to 60 year-olds – equivalent to 6.3 million people – don't manage a brisk ten-minute walk even once a month.
Heart failure affects about 900,000 Britons and up to 6 million Americans. It happens when the heart is unable to pump blood properly - reducing the body's oxygen.
Triggers include coronary heart disease, high blood pressure, heart rhythm problems and cardiomyopathy – which are conditions that affect the heart muscle.
Last year a UK study found men and women living with the condition have a higher risk of death than those with the most common cancers.
It is the leading cause of hospitalisations in those over 65. Risk factors also include high cholesterol, diabetes, smoking and a family history.
Co-author Dr Roberta Florido added : "The population of people with heart failure is growing because people are living longer and surviving heart attacks and other forms of heart disease.
"Unlike other heart disease risk factors like high blood pressure or high cholesterol, we don't have specifically effective drugs to prevent heart failure, so we need to identify and verify effective strategies for prevention and emphasise these to the public."
Drugs such as beta blockers and ACE inhibitors to reduce the heart's workload are usually prescribed only after disfunction is already there.
Examining impact over time
Dr Florido said previous research has suggested people who are more physically active are less vulnerable.
But little was known about the impact of changes in exercise levels over time.
Now it has been shown exercising in middle age - even after leading a sedentary existence - decreases the risk.
But being active before slowing up increases it.
To address those questions, the researchers used data already gathered from 11,351 participants in the federally funded, long term Atherosclerosis Risk in Communities (ARIC) study.
They were recruited from 1987 to 1989 in North Carolina, Mississippi, Minnesota and Maryland and had an average age was 60.
They were monitored annually for an average of 19 years for heart attack, stroke and heart failure using telephone interviews, hospital records and death certificates.
Over the course of the study there were 1,693 hospitalisations and 57 deaths due to heart failure.
At the first and third visits - six years apart - each participant filled out a questionnaire about physical activity levels which were rated as poor, intermediate or "recommended" in line with guidelines issued by the American Heart Association.
The latter is at least 75 or 150 minutes of vigorous or moderate exercise a week. Over four-in ten (42%) said they performed this.
This compared to 23 per cent who were in the intermediate bracket of one to 74 or 149 minutes of vigorous or moderate physical activity.
Just over a third (35%) managed none - placing them in the poor category.
Between the visits 24 per cent increased their exercise levels, 22 per cent reduced it and 54 per cent stayed the same.
Those with recommended activity levels at both the first and third visits showed the highest associated heart failure risk decrease - 31 per cent compared to those who were consistently poor.
Heart failure risk fell by about 12 per cent in the 2,702 who rose from poor to intermediate or recommended - or from intermediate to recommended.
On the other hand it increased by 18 per cent in the 2,530 whose activity reported decreased compared to those whose levels were consistently recommended or intermediate.
According to the American Heart Association, fewer than 50 per cent of Americans get recommended activity levels.
The study was published in the journal Circulation.