Physical Activity Program for TRD

Inclusion Criteria: 1. Sedentary adults (engage in less than 60 minutes of moderate-to-vigorous PA per week) between the ages of 18 and 65 years, inclusive, capable of giving informed consent. 2. Participants meeting diagnostic criteria for major depressive disorder without psychotic symptoms according to the Diagnostic and Statistical Manual of Mental Disorders - 5th Ed. and currently experiencing a major depressive episode (MDE) as confirmed by the Mini International Neuropsychiatric Interview (MINI). 3. A Montgomery-Åsberg Depression Rating Scale total score of ≥ 7 at screening (mild-to-severe MDE). 4. Failure of at least two trials of antidepressant therapy of adequate dose and duration during the current episode as established by the Antidepressant Treatment History Form and self-report. 5. Receiving treatments congruent with Canadian Network for Mood and Anxiety Treatments guidelines with no changes to treatments one month before screening (28 days). Exclusion Criteria: 1. Current symptoms of mania, hypomania, mixed episodes, or psychosis. 2. Have received a diagnosis of alcohol or a substance use disorder within the past 3 months or as confirmed by the MINI. Other secondary psychiatric comorbidities (e.g., anxiety disorders, trauma-related disorders, etc.) will not be excluded. 3. Pregnant females. 4. Acute risk for a cardiovascular event (i.e., cardiovascular event in the past within the past 12 months). 5. Have any medical contraindications to exercise according to the Physical Activity Readiness Questionnaire. 6. Self-reported balance, gait, or locomotion difficulties that would preclude participation in a PA program. 7. Have any other condition that, in the opinion of the investigator(s), would adversely affect the subject's ability to complete the study or its measures. 8. Have exercise-induced asthma. 9. Taking medication that interferes with heart rate response to exercise, such as beta blockers. 10. Do not own a smartphone. 11. Do not have reliable access to the Internet. 12. Have previously received intravenous ketamine treatment in the last 2 months. Non-English-speaking individuals are excluded because the ability to communicate study information, answer questions accurately and completely about the study, and obtain consent are necessary.