Magic words:
Osteoporosis and knees.

Hard to add walking as a safety factor on sidewalks when one might have balance issues with knee arthritis or have a hard time walking long distances.

I just got a walking treadmill, 48 inches long with arms along the entire sides for balance.

I have it set on the lowest speed and will very slowly increase the time on it.
This way, I am getting weight bearing exercise in a safe environment and can use all year.