The purpose of this study is to determine whether vigorous exercise training 4-5 days/week for 2 years in sedentary middle aged men and women (ages 45-64) will improve cardiac and vascular compliance to a degree equivalent to life-long exercisers and the sedentary young. Sedentary aging is associated with impaired diastolic function, which can lead to heart failure. However, if exercise training can be implemented early enough in life while cardiovascular plasticity still exists, then functional capacity can be maintained, preventing heart failure.