18 months out from 2 level lumbar fusion. still having muscular issues from tightness, weakness etc. is it realistic to think i can strengthen back muscles at my age(64)?currently walking, yoga, isos.

Yes: The general use or lose it phenomenon is quite true at any age. Regardless of injury or surgery, weakness is a process of nerve injury. Nervous tissue takes a very long time to heal, however if the nerve does not get any signals to move a muscle from you then that muscle will atrophy. That is difficult to overcome but not impossible. Keep activating those muscles to preserve your strength!

Back: Pt and exercises are good no matter what age. Those exercises will strengthen and improve support which will help you now and in the long-term.

Back surgery: Postoperative muscle spasm can be alleviated conservatively with exercise, physical therapies, hydrotherapies, massage therapies, or even with topical otc pain ointment. Progressive muscle weakness warrants a comprehensive neurological evaluation. Perhaps, consider growth hormone therapy - but discuss this issue with your surgeon, primary care provider, or endocrinologist.