From a recent study:
Cancer is the second leading cause of death in the United States, accounting for 1 in 4 deaths annually. It has been estimated that one third of all cancer deaths are related to overweight/obesity, physical inactivity, and poor diet. Epidemiological studies have shown that regular physical activity reduces the risk for some types of cancer as well as postdiagnosis mortality.
Let’s break it down. Physical activity reduces risk of:
Breast cancer: by at least 20%. Higher levels of recreational physical activity especially demonstrated greater risk reduction.
Colon cancer: by 20% or more.
Endometrial cancer: about 30%.
And for the manpeople, prostate cancer: exact risk reduction unclear; however physical activity may protect against the more aggressive forms of the disease.
Zoeller, Robert F. “Lifestyle in the Prevention and Management of Cancer: Physical Activity”. American Journal of Lifestyle Medicine, Vol. 3, No. 5 (2009): 353-361.