Quote from CancerSupportivecare.com

"Many medical professionals, fitness professionals and physically active cancer survivors confirm that exercise (engaged in moderate levels) allows the organ systems to positively adapt and improve metabolic efficiency, thereby allowing for more intensive cancer treatments, fewer side effects and better rest/sleep patterns.

Exercise need not be intense to promote these benefits. As a matter of fact, a significant number of all cancer patients exhibit muscular weakness, decreased functional capacity and fatigue prohibiting them from previous intensive exercise activities. Survivors often feel more lethargic during certain points of their chemotherapy and/or radiation cycles. This is very normal and indicates that the body and the cancer are being affected by treatment. Allowing the body time to heal or to rest is as important, at times, as building muscle or enhancing cardiovascular strength and endurance. Listen carefully to your body when engaging in any physical activity and act accordingly. It is best to error on the conservative side when determining the time, type and intensity of any exercise activity, particularly during any treatment you may be involved in."

Please visit the above site for more information on how exercise can help coping with cancer.