While you are waiting for Sqad, most hospitals that do cancer treatments have a specialist oncology pharmacist. The medical team may be able to put you in touch as they are often able to give a more accurate picture of the likelihood of side effects. It is likely that supporting bone health with supplements and exercise of some kind will be recommended.
The most common side effect is the return of hot flushes, and they are usually less severe than during menopause.
The reduction in the risk of the cancer coming back is well documented but it is a balancing act. If it is any help my mother is in her eighties and has taken letrozole for two years with no problems apart from brief visits to the tropics as she calls it