Walking is great exercise, but as with all exercise after a while you stop seeing as much of an improvement as you did when you first started. You need to step up your aerobic activity in order to continue to see an improvement.

I can't jog for extended periods of time or my body starts to complain, especially my knees. What I can do is jog for short periods, and I do mean short. My goal is to increase my heart rate so that I am slightly out of breath, but not so out of breath that I can't carry on a conversation.

I either time my jog keeping it to 60 seconds, or I jog between two specific points, for example from one corner to the next. I am not a runner and never will be, but I do want to get the most out of my exercise and adding some running helps me do that.