During this time, participants will exercise, keep a daily exercise log detailing distance walked (routes will be measured and mapped for participants), time spent in aerobic activity, degree of intensity (rating of perceived exertion), general level of affect during exercise, resting and exercise heart rates. Participants will participate in the MRI/fMRI, physiological (cardiorespiratory) and psychosocial testing prior to the beginning of the fitness training intervention, following 6 months of fitness training, and at the conclusion of the 1 year of fitness training.