Extended Endocrine Therapy for Premenopausal Women With Breast Cancer

* Participants will be given a monthly intramuscular injection of leuprolide. Six to eight weeks after the first leuprolide injection, participants will be instructed to begin taking letrozole pills by mouth at home each day. * During this research study, participants will also be offered injections of zoledronic acid to preserve bone mineral density. If desire, these injections will be given every 6 months for a total of 4 injections during the course of the research study. * The treatments in this research study may increase the risk of bone density loss, so vitamin D and calcium supplements will be recommended to participants. * The following tests and procedures will be performed during the research study: Physical Exam (every visit); Blood tests; Urine test (beginning and end of treatment); Imaging (annual mammograms); Bone Density (DXA) Scan (beginning and end of treatment); Questionnaires (throughout the study). * Participants will be on this study for about two years.