Exercise intervention: The exercise will take place in an exercise facility at the Geriatric Research Center on the campus of the Wake Forest University/Baptist Medical Center. Two study personnel, including at least one trained technician, will supervise all exercise sessions. Emergency equipment will be kept on site during exercise sessions. Blood pressure and heart rate (HR) will be measured before each exercise session and subjects will warm-up by walking for 3-5 min at a slow pace and will then walk at an intensity of 65-70% of heart rate reserve (HRR, assessed during the VO2max test). The duration of walking exercise will progress from 15-20 mins at 50% HRR the 1st week to 30 mins at 65-70% HRR by the end of the 6th week and thereafter. Each walking session will end with a 3-5 min cool-down followed by 5 min of large muscle flexibility exercise.