Lactic acid, a byproduct of muscle metabolism (working muscles) is often blamed as the cause of muscle soreness 24-48hours after working out. Lactic acid itself is removed very quickly from the body (generally beginning in seconds and with complete removal within around 30minutes). It will often causes a “burning” sensation during exercise, but is relieved within minutes of stopping that particular exercise. It is not the cause of soreness a few days after your workout. Current research suggests that this muscle pain and soreness 24-48h post workout (delayed onset muscle soreness or DOMS for short) is due to microtrauma of the muscle fibers contractile elements (actin and myosin) and membranes (sarcolemma). This microtrauma causes inflammation and release of chemicals from damaged tissue which stimulates pain receptors. This also is an increase of blood flow to the area leading to swelling of the muscles which also stimulates pain receptors located in nerves.
A brisk walk 2-3hours per week lowers risk of death by 45% in breast cancer patients.
If that isn’t reason to exercise, I don’t know what is. In a study of nearly 1000 women with breast cancer, compared with women who were inactive both before and after diagnosis, women who increased physical activity after diagnosis had a 45% lower risk of death
Influence of Pre- and Postdiagnosis Physical Activity on Mortality in Breast Cancer Survivors: The Health, Eating, Activity, and Lifestyle Study ML Irwin, AW Smith, A McTiernan, R Ballard-Barbash, K Cronin, FD Gilliland, RN Baumgartner, KB Baumgartner, L Bernstein (J Clin Oncol, 2008)