This is interesting stuff, so have a look at this free editorial and the article (access, $32) in the current issue of the Journal of the National Cancer Institute. Here's an excerpt from the editorial by Edward Giovannucci, MD.
Given the promise of a role of physical activity in reducing cancer-specific mortality, future research involving observational studies, interventions, and mechanistic studies should be a high priority. Few other leads have shown as much promise as physical activity in extending the lives of cancer survivors. Many treatments may increase survival, but at a cost of quality of life; physical activity may not only extend life but may also enhance its quality. Even though direct effects of physical activity on cancer are not definitively proven, given that physical activity is generally safe, improves quality of life for cancer patients, and has numerous other health benefits, adequate physical activity should be a standard part of cancer care.