I too was a fit and active woman diagnosed suddenly when I was 60, nearly 5 years ago now. Welcome to the forum.
In the UK it is standard practice to encourage patients who are younger than their early 70s to have stem cell transplants, if they are fit enough. However in the USA there is no such standard practice and some consultants rarely advise SCT in the light of the range of targeted drugs now. If you do regret not having a SCT it is worth asking your Dr if it would still be possible to collect stem cells and to do the procedure. Lenalidomide is known to impair stem cell production so this may not be possible. However as rosary has pointed out, not all myelomas respond to stem cell transplants, and some patients do not benefit, or not for long.
My guess is that you made the best decision that you could at the time, with the information that you had. Try not to regret the decision you made, no one knows for certain how myeloma will respond to any specific treatment.
New drugs such as bispecific antibodies may become game changers for us with myeloma, in the meantime there are a number of good drugs approved by NICE for NHS myeloma treatment. By the time you come to need new treatments, novel treatments may be approved & available which may outweigh the collateral damage done by melphalan during SCT. It may turn out that those of us who had SCTs then feel that you made the right decision.
Best wishes, Jane