 AIM 3: Preliminarily evaluate i the biological mechanisms by which EXER affects brain health and cognition, and ii the individual difference factors that potentially moderate EXERs effects. Hypotheses: i Persons in the enhanced physical activity group will exhibit significant increases in circulating neurotrophins and improved cardiorespiratory fitness, and ii the beneficial effects of EXER will be more pronounced for participants with decreased sedentary behaviors outside of the intervention measured via accelerometry. 