More than 20 million people in the UK are physically inactive, according to a report by the British Heart Foundation.
The charity warns that inactivity increases the risk of heart disease and costs the NHS around £1.2bn each year.
Harriet Mulvaney experienced a heart attack at 44 and decided to make changes to her lifestyle.
"Looking back on it now I would say I was very inactive. I thought I was active but actually I think I was just busy," she says.
Women are 36% more likely than men to be classified as physically inactive - 11.8 million women compared with 8.3 million men.
The report defines "inactive" as not achieving the government guidelines for physical activity of 150 minutes of moderate intensity physical activity a week and strength activities on at least two days a week.
Exercise guidelines for adults aged 19 to 64
At least 150 minutes of moderate aerobic activity or 75 minutes of vigorous aerobic activity every week
Strength exercises on two or more days a week that work all the major muscles
Break up long periods of sitting with light activity
What counts as moderate aerobic activity?
Walking fast, water aerobics, riding a bike on level ground or with few hills, doubles tennis, pushing a lawn mower, hiking, skateboarding, rollerblading, volleyball, basketball
What counts as vigorous activity?
Jogging or running, swimming fast, riding a bike fast or on hills, singles tennis, football, rugby, skipping rope, hockey, aerobics, gymnastics, martial arts
What activities strengthen muscles?
Lifting weights, working with resistance bands, doing exercises that use your own body weight, such as push-ups and sit-ups, heavy gardening, such as digging and shovelling, yoga.
What activities are both aerobic and muscle-strengthening?
Circuit training, aerobics, running, football, rugby, netball, hockey