You are very wise to consult with your doctor first. I know you have had some health issues - wonder if anything you had merits a session or two with a physical therapist? Your doctor could write you a script.
You should seriously consider a few sessions with a physical trainer. Folks can give you good-meaning advice, but you can't do the wrong thing or too much of the thing that can be right for others. The last thing you need is to injure yourself.
Please consider that aerobic exercise is great, but there sometimes can be a lack of strengthening your core. That is so crucial as you age. I never strengthened mine, suffered a lot as a consequence, and am now completing many sessions of PT in a warm water pool. My doctor is seeing results, I am seeing results, and I plan to continue my exercises in a pool once therapy is done. My pain is so much better. I recently spoke to another lady in my PT pool therapy, a senior like me, who walked a great deal, but she is also suffering from very poor core muscles.
I really hope you and some others can learn from my mistake. As my PT has confirmed, there was a time when everyone thought aerobic exercise was all you needed. While walking is wonderful, please get some advice appropriate to your age and condition to work on that core. My best wishes to you and my admiration for your desire to get stronger.