Tens of thousands of lives could be saved each year in the UK if people got off the sofa and stretched their legs more, say charities.
The "Walking Works" report by the Ramblers and Macmillan Cancer Support said walking was a free activity which could transform people's health.
Being physically active decreases the odds of heart problems and stroke.
But it also makes a difference in other conditions such as type 2 diabetes, Alzheimer's disease and many cancers.
The latest report said that if everyone, in England alone, did the recommended 150 minutes of moderate physical exercise every week it would:
- Save 37,000 lives each year
- Prevent 6,700 cases of breast cancer
- Stop 4,700 people getting colorectal cancer
- Lead to nearly 300,000 fewer cases of type 2 diabetes.
The two charities run the Walking for Health programme in an attempt to get more people up on their feet.
Benedict Southworth, chief executive of the Ramblers, said: "We're facing a serious crisis of inactivity, but there is a simple solution,
"We need to see greater investment in initiatives which support and promote walking as the most accessible and affordable way for people to get active."
Ciarán Devane, chief executive of Macmillan Cancer Support, said: "For cancer patients, being active can help manage some of the debilitating consequences of treatment and can even help reduce the chance of some cancers returning.
"Inactivity is a nationwide epidemic that must be tackled now before it is too late."
Public Health England said inactivity had "life threatening consequences".
Its director of health and wellbeing Prof Kevin Fenton said: "Inactivity increases the risk of serious illnesses like type 2 diabetes, heart disease and certain cancers.
"It makes it more likely that people will be overweight or obese. Supporting people to get active through walking can be a major part of the solution."