Walking is as good as running when it comes to reducing a middle-aged woman’s risk of suffering a heart attack or stroke, according to the results of a large study sponsored by the federal government.

Brisk walking for about 2 1/2 hours a week or an equivalent amount of more strenuous exercise cut the risk of heart disease and stroke by about one-third, the researchers found after tracking about 74,000 women for six years.

The results suggest that the benefits of exercise are within reach of virtually every American woman and don’t require equipment, organized sports or painful exertion.

“No pain, no gain is an outdated notion,” said JoAnn E. Manson, chief of preventive medicine at Brigham and Women’s Hospital in Boston and the lead author of the study, which appears in today’s New England Journal of Medicine. “Exercise doesn’t need to be strenuous or uncomfortable moderate exercise will provide the lion’s share of the health benefit.”

A separate report in the same journal found that the amount of regular exercise girls get falls off dramatically as they move through their teenage years, dropping to practically zero in many cases, especially among blacks.

By the time they were 16 or 17, more than half of the black girls in the study and nearly a third of the white girls reported they got no regular exercise at all outside school.

With obesity at epidemic levels, “it’s a cause for alarm,” said Dr. Sue Y.S. Kimm of the University of Pittsburgh School of Medicine. “We cannot sit complacent anymore.”

The girls’ decline in physical activity was affected by lower levels of parental education, heavier weight, smoking and pregnancy. Girls with better-educated parents may be better informed and more encouraged to exercise, the researchers suggested.