• Women

• Able to provide written informed consent

• Able to speak, read, and understand English or Spanish

• Postmenopausal (including concurrent use of ovarian suppression)

• Diagnosis of ER+ and/or PR+, hormone estrogen receptor-2 (HER-2)- or HER-2+ metastatic or locally advanced unresectable breast cancer

• Initiation of first-line or second-line endocrine therapy in combination with a CDK4/6 inhibitor (e.g., palbociclib, ribociclib, or abemaciclib), with or without hormone estrogen receptor-2 (HER2)-directed therapy (e.g., trastuzumab ± pertuzumab), or in combination with both a CDK4/6 inhibitor and a Phosphoinositide 3-kinase (PI3K) inhibitor, within the past 90 days.

∙ Prior use of a CDK4/6 inhibitor in the adjuvant setting is permitted; participants must be enrolled within 90 days of starting a CDK4/6 inhibitor for the first or second time in the metastatic or locally advanced setting.

‣ Participants who switch from one CDK4/6 inhibitor to another due to toxicity or insurance constraints remain eligible, provided enrollment occurs within 90 days of initiating the first or second-line CDK4/6 inhibitor in the metastatic or locally advanced setting.

• Having not consistently (not equal to or more than 50% of the time) engaged in more than 90 minutes of moderate or 45 minutes of vigorous physical activity per week over the past 3 months

• Having not consistently engaged in resistance training 2 or more days per week over the past 3 months

• Having not consistently (not equal to or more than 50% of the time) engaged in structured fasting (i.e., not regularly fasting for ≥12hr/night) for the past 3 months

⁃ Approval from a medical oncology provider to participate.