Another reason to engage in physical activity has been added to the stack of good reasons you may be trying to ignore. Recent research shows that exercise can decrease the risk of postmenopausal breast cancer.

According to a study published in Cancer Epidemiology, Biomarkers & Prevention, postmenopausal women who completed the equivalent of four hours of walking per week over four years showed lower riske for invasive breast cancer.

"Twelve MET-h [metabolic equivalent task-hours] per week corresponds to walking four hours per week or cycling or engaging in other sports two hours per week and it is consistent with the World Cancer Research Fund recommendations of walking at least 30 minutes daily," said Agnes Fournier, lead author of the study from the Centre for Research in Epidemiology and Population Health at the Institut Gustave Roussy.

"So our study shows that it is not necessary to engage in vigorous or very frequent activities; even walking 30 minutes per day is beneficial."

Researchers looked at data from 59,308 postmenopausal women in the French component of the European Prospective Investigation into Cancer and Nutrition study. The duration of the study was approximately 8.5 years. They observed 2,155 incidents of primary invasive breast cancer between questionnaire administrations.

Women who had the recommended amount of physical activity had a 10 percent decreased risk. However, researchers found that even if women were that active five to nine years prior, if they did not keep up with 12 MET-h per week of activity, they did not exhibit a decrease in risk.

How long after physical activity starts or stops do the benefits of exercise begin and end? Fournier said researchers aimed to answer this question in the study.

"Our study answers these questions," she said. "We found that recreational physical activity, even of modest intensity, seemed to have a rapid impact on breast cancer risk."

Researchers observed the benefit of physical activity independent of factors such as body mass index, weight gain and level of activity from over five years ago.

Fournier said this should encourage postmenopausal women who already exercise to continue to participate in physical activity and to encourage women who don't already have regular exercise regiments to start in order to decrease their breast cancer risk.