The HHS department produced some guidelines for health recently. The guidelines sound like they fell out of a fitness enthusiast forum. :-) They recommend something like 2-5 hours of walking (or other aerobics) per week and 2+ days of lifting. I quote two of their senior officials in an article they put out in JAMA:
It is difficult to convey the health benefits of physical activity in a way that does not sound like it is a "cure all". Physical activity aids in reducing the risk of myriad diseases, including obesity, type 2 diabetes, cardiovascular disease, dementia, and 8 forms of cancer (bladder, breast, colon, endometrium, esophagus, kidney, lung, and stomach). Physical activity improves sleep and physical function, prevents injury from falls, and is beneficial as an adjunct to pain management, particularly for persons with osteoarthritis or other rheumatic conditions. Overall, individuals who meet the PAG recommendations could lower their risk of premature death by 33%, compared with those who are not physically active."""