A moderate increase in fitness level can decrease a man's risk of dying by between 50 and 70 percent, according to a study conducted by the Exercise Testing and Research Lab at the Veterans Affairs Medical Center in Washington, and published in the journal Circulation.

"It is important to emphasize that it takes relatively moderate levels of physical activity - like brisk walking - to attain the associated health benefits," said researcher Peter Kokkinos. "Certainly, one does not need to be a marathon runner. This is the message that we need to convey to the public."

Researchers studied more than 15,000 male U.S. veterans, 6,749 black and 8,911 white. The men had been given standardized treadmill tests, in which they were encouraged to walk until they were tired, then monitored for an average of 7.5 years each.

Depending on their treadmill test performance, the men were classified as "low fit," "moderately fit," "highly fit" or "very highly fit." Compared with men in the "low fit" category, men in the "highly fit" category had a 50 percent lower risk of death during the study period, while men in the "very highly fit" category had a 70 percent lower risk of death.

During the study period, 44 percent of the "low fit" men died, compared with 30 percent of the "moderately fit" men, 15 percent of the "highly fit" and 8 percent of the "very highly fit."

According to Kokkinos, moving from "low fit" to "highly fit" would not be hard to achieve.

"All you need is between two and three hours of brisk walking a week. You can spread that out between four and six days a week," he said.

Achieving "very highly fit" would take only 30 minutes of brisk walking, five or six days a week, for several months.

The study is one of the largest to date that shows a link between exercise and longer life.