A study by Hildebrand et al, published in the journal Cancer Epidemiology, Biomarkers & Prevention, report that in a study of over 70,000 post-menopausal women, the most active women (those reporting over 42 MET hours per week, or approximately 14 hours per week of brisk walking) experienced 25% lower risk of breast cancer than the least active (less than three hours per week of walking at an average pace).

Forty-seven percent of women reported walking as their only recreational activity; among these women, a 14% lower risk was observed for ≥7 hours/week relative to ≤3 hours/week of walking.

Associations did not differ by estrogen receptor status, BMI, weight gain, or post-menopausal hormone use. Sitting time was not associated with risk. These results support an inverse association between physical activity and postmenopausal breast cancer.

For a chart of various physical activities in terms of their METs (a way of measuring the exercise equivalents) see this chart put out by Harvard Health Publishing.