Physical Activity, Proliferation and Immune Markers in Benign Breast Tissue

Inclusion Criteria: * Written informed consent prior to any study-related procedures * Premenopausal women, defined as: * having regular menstrual cycles * age ≤ 50 years with an intact IUD * history of hysterectomy without oophorectomy * Heterogeneously dense or very dense (BIRADS 3 or 4) breast tissue on mammogram within the last 3 years * Physically inactive; engaging in \<90 minutes of moderate or vigorous intensity PA per week * No prior history of breast cancer * At least 18 years old * Physically able to exercise * English speaking and able to read English Exclusion Criteria: * Self-reported inability to walk 2 blocks (at any pace) * Prior history of breast cancer; prior DCIS is allowable as long as participant is not taking endocrine therapy and has at least 1 breast that has not been irradiated. * On oral contraceptives; Mirena IUD is acceptable * Cardiovascular, respiratory or musculoskeletal disease or joint problems that preclude moderate physical activity. Examples would include unstable angina, recent myocardial infarction, oxygen-dependent pulmonary disease, and osteoarthritis requiring imminent joint replacement. Moderate arthritis that does not preclude physical activity is not a reason for ineligibility. * Psychiatric disorders or conditions that would preclude participation in the study intervention (e.g. untreated major depression or psychosis, substance abuse, severe personality disorder)