People, physically inactive, face a considerably higher risk to acquire non-communicable diseases, especially when advanced in years. This is a generally accepted fact concerning coronary heart diseases, strokes, hypertension, diabetes or osteoporosis.
However, this applies to various types of cancer, too. It is estimated that 22-33% of colorectal cancers and 5-12% of all cases of breast cancer could be avoided by increased physical activity and motility.

Whereas most individual studies did not result in significant data, meta-analysis have clearly confirmed that physical activity is associated with reduced risk of cancer affection, best evidence being available for colorectal, lung and breast cancer: Cohort and case-control studies suggest that for women and men, physical activity, both, occupational and during leisure time protect against breast and colon cancer. Generally spoken, the more exercise, the better. Data from 19 cohort studies showed that physical activity is positively correlated with reduced risk of colon cancer in both, males and females (r^2 = 0.79 and 0.71, respectively). For rectal cancer, no significant correlation could be observed.

For further reading visit:
Tardon A et al. Leisure-time physical activity and lung cancer: a meta-analysis. Cancer Causes Control. 2005 May ; 16(4): 389–397.
Samad AK et al. A meta-analysis of the association of physical activity with reduced risk of colorectal cancer. Colorectal Dis. 2005 May;7(3):204-13.
Cherkas LF. The association between physical activity in leisure time and leukocyte telomere length. Arch Intern Med. 2008 Jan 28;168(2):154-8.
Thune I. and AS Furberg. Physical activity and cancer risk: dose-response and cancer, all sites and site-specific. Med. Sci. Sports Exerc., Vol. 33, No. 6, Suppl., 2001, pp. S530-S550.