Aim 3: Examine whether the 12-week combined physical activity and pharmacotherapy intervention changes the structural morphology in specific subfields of the hippocampus. Hypothesis 1: The medication intervention will increase hippocampal volume in the dentate gyrus and carbonic anhydrase I (CA1), but combining aerobic exercise with pharmacotherapy will magnify the effects of exercise. Hypothesis 2: The effect on hippocampal volume will be larger for older versus younger adults Aim 4: Explore how the combination of pharmacotherapy and exercise (compared with pharmacotherapy and TAU) influences a range of brain and behavioral outcomes, including resting state brain dynamics, MR spectroscopic measures of GABA, sleep efficiency, and cognitive performance. Hypothesis 1: Antidepressant pharmacotherapy will alter resting state networks, increase GABA levels, and improve sleep efficiency and cognitive performance - but these effects will be greater when combined with an aerobic exercise intervention. Hypothesis 2: These effects will be moderated by age such that the effects will be greater in older adults, supporting a dissociation between depression in younger and adults, and providing justification for fully powered study to explore these models and treatment-predictive biomarkers.