If you maintain or improve your fitness level, even if your body weight has not changed or increases, you can reduce your risk of death. This data is according to research reported in the journal Circulation in December of 2011 on 14,000 men of average age 44 over 6 years. For every MET (the energy expended during exercise) increase over 6 years of a 19% reduction in MI and CVA, and a 15% decrease in all cause mortality. Body mass index (BMI), which is a measurement of body fat based on height and weight, was not related to death risks nor were changes in percent body fat or weight.
Exercise is the most potent anti-aging strategy known to science. If there was a pill that could reproduce just 10% of what exercise could, it'd be a blockbuster drug! It has powerful effects on our hormones, such as growth hormone, DHEA, testosterone, thyroid hormone and many others as well as neurotransmitter levels, depression, diabetes, hypertension, heart attacks, strokes, and so much more.
Even a couch potato will benefit from lower mortality and increased quality of life if they exercise just one hour a week. Lastly, the intensity of exercise matters, and interval training is the most beneficial.