So-called "green exercise" provides health benefits worth £2.2 billion a year to adults in England, according to research.
Researchers analysed data from the world's largest study on recreational visits to natural places - such as parks, woods and beaches.
They estimated that more than eight million adults in England engage in green exercise each week, resulting in over 1.3 billion green exercise visits a year.
Green exercise was defined in the study as nature-based activities of moderate to vigorous intensity and lasting more than 30 minutes.
Examples included dog walking, running, horse riding, outdoor swimming and mountain biking.
Because physical activity needs to be regular and sustained to benefit health, the team focused on those who reported regularly meeting Government guidelines for physical activity of five 30 minute sessions each week.
Researchers from the University of Exeter Medical School and Public Health England then worked out what proportion of these people's weekly physical activity took place in natural settings and estimated the benefits to health associated with their levels of green exercise if sustained across the year.
Dr Mathew White, who is the lead author of the research, said: "We've known for a long time that regular physical activity is good for health and reduces the burden on health services.
"We have now worked out approximately how much physical activity regularly takes place in England's natural environments and how much this benefits adult health across the population.
"Ultimately, these benefits will translate into savings for the NHS - highlighting the need to both maintain and promote our natural environments for exercise and health."
The data analysed was from Natural England's "Monitor of Engagement with the Natural Environment" survey, which has more than 280,000 participants and has been running for the last six years.
The survey outcomes are part of England's official national statistics and people are sampled each week of the year, and in every region of the country, to reduce seasonal and geographical biases.
Dr Angie Bone, of Public Health England and co-authored the work, outlined some of the broader implications of the findings.
"Our parks, gardens, coasts and countryside play a vital role in improving health in this country, inspiring millions of us to get active outdoors every year," she said.
"Evidence suggests that access to good quality green space is linked to feeling healthier, a lower body mass index and decreased levels of obesity, and improved mental health and wellbeing.
"This research highlights the positive impact getting outdoors has on our health, emphasising the importance of both promoting exercise outdoors to a wider population and maintaining the quality and accessibility of the nation's parks and wild places."
:: The study Recreational physical activity in natural environments and implications for health: A population based cross-sectional study in England is published in the journal Preventive Medicine.