As a runner and as someone who has gone to the gym for years, and as someone who has has physical therapy at times for various injuries, I'd like to suggest something. The lower body muscles can easily get weaker as you age. Plain old walking isn't enough.

Some ideas for easy exercises you can do at home with no equipment needed:

1. Balance on one foot. Stand next to a table or something in case you need to steady yourself. Balancing helps strengthen all those little muscles in your foot and ankle. It also helps... well, your sense of balance! I try to hold it for 60 seconds, each foot. I do it a couple of times a week.

2. Squats. I don't mean the exercise in the gym where you have a barbell on your shoulders. All you need to do is stand with your back on the wall, and slide down just a bit until you feel it in your thigh muscles. You can even use one of those inflatable exercise balls and put that between your back and the wall. The muscles right above your knee are not easy to target unless you either do this exercise or you walk up hills.

3. Walking up hills, besides the thighs, also strengthens your glutes and hips. A park with a trail that isn't completely level is great. Or, if there's a street near you that is a hill, walk up and down it a few times.