Optimizing a mHealth Physical Activity Intervention With Mindful Awareness Lessons in Breast Cancer Survivors

Inclusion Criteria: For BCS * Female, age 18 years or older * Stage I-III breast cancer diagnosis within the past 5 years * Diagnosis of cancer between 18-39 years and within the past 5 years * ≥3 months post-completion of primary treatment (i.e., surgery, chemotherapy, and/or radiation); may still be undergoing endocrine or hormone therapies * Self-report engagement in \<60 min/week of MVPA * Score of 0 on all positive statement(s) of the Physical Activity Readiness Questionnaire (PAR-Q)18 or willingness to obtain medical clearance from a primary care physician or oncologist * Internet access and smartphone ownership * Fluency in spoken and written English * Willingness to find a buddy (someone who knows about their cancer and participation in this study and who is willing to participate by supporting them during the 24-week study), if assigned to that condition For "Buddies" * Friend, co-worker, caregiver or family member of a participating BCS * Age 18 years or older * Score of 0 on all positive statement(s) of the Physical Activity Readiness Questionnaire (PAR-Q)18 or willing to obtain medical clearance from a primary care physician * Internet access and smartphone ownership * Fluency in spoken and written English * Willingness to share their Fitbit data with the study team Exclusion Criteria: * Absolute contraindications to exercise (i.e., acute myocardial infarction, severe orthopedic conditions), metastatic disease, or planned elective surgery * Currently pregnan…