Metastatic breast cancer (MBC) is an incurable disease. Maintaining optimum quality of life is a major goal of care. There is a strong body of evidence that exercise can reduce or manage fatigue, depression and insomnia in breast cancer patients; however, the evidence base is overwhelmingly in early stage cancer patients. The purpose of this study is to see if a home-based, self-directed walking program can have similar benefits in women with an MBC diagnosis. The primary objective is to evaluate whether engagement in physical activity will reduce fatigue during active treatment for MBC (baseline to 3 months); this is the primary endpoint. Secondary objectives pertain to feasibility of recruitment and retention of study participants and measuring changes between baseline, 3 months and 6 months in additional quality of life measures. Exploratory analyses pertain to changes in p16INK4a levels and sarcopenia between baseline and 3 months. The design is a single arm intervention trial in 30 patients. Findings from this study will provide preliminary data for a grant application to test the physical activity intervention in a randomized controlled trial (RCT) in a large sample of women with metastatic breast cancer.

Inclusion Criteria:

• Female
• Age 21 or older
• Metastatic breast cancer (MBC) diagnosis - Stage IV
• Receiving treatment regimen for MBC, but no more than 3 previous chemotherapy or biologic regimens
• Approval from the treating clinician to engage in moderate-intensity physical activity.
• Patient-assessed ability to walk and engage in moderate physical activity
• Proficient in English.
• IRB approved signed written informed consent.
• Willing and able to do all study requirements.

Exclusion Criteria:

• One or more significant medical conditions or other issues that in the physician's judgment preclude participation in the walking intervention.
• No more than 3 prior chemotherapy regimens.