Breast cancer (BC) is the most prominent cancer amongst women, but fortunately, early diagnosis and advances in multimodality treatments have improved patient survivability. Cancer survivors, however, experience increased biological ageing which may accelerate other co-morbidities. Exercise intervention is a promising clinical adjuvant approach to improve BC patients' physiological function, recovery from treatment, and quality of life. However, the effects of combined aerobic and strength exercise training on biological ageing in BC patients have not been studied. The Breast Cancer Exercise Intervention (BREXINT) Pilot Study will evaluate the effects of a 24-week combined aerobic and strength exercise intervention against usual care in 50 BC patients' post-treatment randomised to either group. The primary outcomes include changes in cardiorespiratory fitness, muscle strength, cancer-related symptoms, and rate of biological ageing following exercise intervention period. The secondary outcomes include habitual physical activity measured with tri-axial accelerometery and supporting questionnaires, including physical activity, food diary, and quality of life questionnaires. This study will identify the effects of combined aerobic exercise strength training on biological ageing in BC patients from Singapore. Results from this study could further support the implementation of regular exercise programmes as routine care for cancer patients.