RecruitingBehavioural interventionResistance and Aerobic Exercise for Prevention in Women With Dense Breasts (REP-D)The goal of this study is to learn more about how exercise might lower the risk of developing breast cancer in women with dense breast tissue by studying changes that occur in breast tissue and blood as a result of participating in an exercise program. The names of the study groups in this study are: * Exercise Training Group * Waitlist Control Group