New to this site and new to exercise!

Welcome aboard, and do remember to take irt slow and steady, Frank

Break yourself into the exercise (I like calling it activity, doesn't sound so grueling!) at your own pace. I never used to do anything then started with a personal trainer. She can tell what I am capable of and increased it when I was ready. Now I can keep up with or out do kids 20 years younger than me. Walking like you said is a great start. Also try stair climbing to build up the cardio. Just go up and down a few times. Then add going backward or sideways to it. Then skip a step. You can combine all the possible ways, then add carrying some sort of weight to it and gradually increase your cardio as you do more flights at one time. When you make it a longer workout maybe after every couple flights stop and do something at the bottom such as bicep curls or toe touches or anything you can think of to make it a more interesting workout. Time goes by fast. Something simple you can do at your own pace.