This pilot study aims to test a model that predicts that enhanced neurotransmitter gamma-aminobutyric acid (GABA) function in reward and affect-regulation central nervous system (CNS) circuits mediates the antidepressant effects of exercise. State-of-the-art magnetic resonance (MR) imaging, cognitive assessment, accelerometry, genetic, and inflammatory biomarkers will be acquired through the coordination of efforts from several established research programs at Western Psychiatric Institute and Clinic. This pilot study will be used as a platform for testing a causal/mediating role of GABA interneurons in reward processing and affect regulation in humans. This pilot study is not powered for testing a full causal model, but rather is intended to test overall feasibility of the intervention and acquisition of measures (see specific aim 1 below). This is a necessary prerequisite for designing a larger more definitive study of the model, which will be a component of a future grant application. Additionally, the data from this study will be used to test the clinical efficacy of exercise as an adjunctive treatment for late life depression (LLD; Specific Aim 2), as well as imaging, cognitive, and sleep aims (Specific Aims 3 and 4).