Inclusion Criteria:
* Patients with histologically-confirmed, HR-positive (ER and/or PR), HER2-negative metastatic breast cancer (MBC)
* Postmenopausal status defined by lack of menses for 2 years, oophorectomy, or medical ovarian suppression
* Receiving first line endocrine-based therapy (tamoxifen, aromatase inhibitor, or fulvestrant; concurrent molecular therapy also allowed)
* Sedentary (i.e., \<60 minutes / week of exercise)
* Age \>18 years
* BMI ≥ 18.5
* Cleared for exercise participation as per screening clearance via PAR-Q+
* Cleared for study participation as per screening consultation with an MSK Exercise Physiologist
* Willingness to comply with all study-related procedures
* Patients with "treated and stable" brain lesions of a duration of ≥ 2 months may be enrolled
Exclusion Criteria:
* Life expectancy \<6 months
* Enrollment onto any other therapeutic investigational study
* Mental impairment leading to inability to cooperate
* Concurrent participation in weight loss or other exercise programs
