Participants in the exercise program will exercise (e.g., walking, running, cycling, stretching, and weight training) for 1 hour at the study exercise laboratory 2 days a week for 12 weeks. If participants cannot come to the laboratory, they may exercise at home, under the guidance of study researchers. Participants will wear a pedometer on select days and keep an activity log to record how far they walk. At the end of the 12-week program, participants will undergo repeat baseline testing. They will also receive a written, personalized exercise program that they can follow on their own. Participants will receive weekly phone calls from study researchers and will attend study visits once a month for 3 months for follow-up and monitoring. Baseline testing will occur again at the end of the 3-month follow-up period.