National guidelines emphasize physical activity as essential to survivorship care, yet many breast cancer survivors perceive themselves as inactive despite meeting recommended activity thresholds. In this ancillary mixed-methods study, participants underwent a 10-day accelerometer-based physical inactivity verification period, followed by surveys and semi-structured interviews if they exceeded 90 minutes of moderate-to-vigorous physical activity (MVPA) per week. Preliminary data from the parent study show unusually high MVPA levels despite self-identification as inactive, suggesting possible measurement reactivity, demand characteristics, peak-end recall effects, or cancer-related worry influencing self-perception. Qualitative interviews explore motivations, barriers, emotional context, and perceptions of what “counts” as MVPA. Quantitative measures include objective activity metrics and validated scales assessing fear of recurrence and cancer-related worry. This ongoing study aims to clarify perceptual discrepancies, improve counseling strategies, and guide the design of future activity-based survivorship interventions.