Effects of Exercise Intervention on Insulin Levels in Breast Cancer Survivors

* All women taking part in this study will take part in an exercise program. Group A will start exercising immediately and Group B wil have a delay period of 16 weeks before starting to exercise so investigators can look at the changes in insulin levels that occur in a women who is not exercising. * The exercise program will be made up of two parts: strength (or weight) training and cardiovascular exercise. The strength exercise will take place at a gym close to Dana-Farber Cancer Institute. Patients will work one on one with a personal trainer twice a week for 50 minutes. During each of these sessions, participants will have a brief warm up period and then will perform a series of Nautilus machine-based exercises under the guidance of the personal trainer. The goal of each session will be to increase strength by increasing weight lifted and repetitions. * Participants will also do cardiovascular exercises on their own at home. Each participant will receive a pedometer and heart rate monitor. Women will be advised to exercise for at least 30 minutes three times per week on their own and will keep a journal recording how long and how intensively they exercised. Participants will receive weekly phone calls from the project manager to review the cardiovascular exercise. * Women will undergo a series of measurements before and after the 16 week exercise intervention. They will be asked to fast for 12 hours prior to each of these measurement sessions. Measurements will include blood testing for insulin and glucose levels. Women will also be weighed, height will be measured, body composition will be computed using a noninvasive bioelectric impedance monitor, and hip/waist measurements will be taken. Group A participants will undergo testing at the time of enrolling in the study and after 16 weeks of exercise. Group B participants will undergo these tests after enrollment, after the 16 week waiting period and at the end of the exercise program. * Participants will also complete two surveys by mail, 3 months and 9 months after completing the exercise portion of the protocol.