We do not stop exercising because we grow old--we grow old because we stop exercising. Dr. Kenneth Cooper, Cooper Institute

     The New York Times Health section had an interesting article this week titled:  "Aging Well Through Exercise."  Previous studies showed that after age 40 people lose 8% of their muscle mass per decade, which accelerates after age 70.  New studies done on athletes ages 40 to 80, however, show that is not the case.  Exercise mitigates those muscle losses.  What was previously thought to be muscle loss due to aging was actually muscle loss due to inactivity.
     More research needs to be done to determine the right types and amounts of exercise; how much strength training vs how much endurance training.  It is good to know, though, that aging can be under our own control, that we can avoid the decline to frailty if we want.

 http://well.blogs.nytimes.com/2011/11/09/aging-well-through-exercise/?ref=health  

     Or my suggestion is that you walk your dog in an area full of squirrels, rabbits, and deer, and combine your endurance training, strength and balance training all at the same time.