Additional Study Information: This study focuses on the effects of aerobic conditioning on cognition and brain activity. Eligible participants will complete an exercise stress test, MRI, and cognitive testing session at the beginning and end of the study. Once participants complete the first set of appointments, they will be randomly assigned to one of two conditions for 12-weeks. Participants could be in an aerobic fitness condition or a control condition. Regardless of condition, participants will also be provided with a pedometer to use daily for the duration of the study.