If you don't have a heart rate monitor yet, I think you should get it. There are a wide range of models with all kinds of functions available these days.
If you do have it, chances are you are wondering how to determine the optimal heart rate for aerobic training. There are many methods and formulas for this purpose out there. I've been using the one by Dr. Philip Maffetone because it seems to be the simplest and most effective one. I learned it from his book Training for Endurance (1996) but it does not seem to be available anymore. However, The Maffetone Method seems to be available and I guess it's a similar one although I've never read it.
Anyway here's how you can quickly calculate your maximum aerobic heart rate: 180 - age. Yes, it's that simple. For example, I'm 45, so the upper limit is 180-45=135. Of course, you may adjust it a little down if you have experienced any health issues lately. I would be cautious to adjust it much higher though, because then you risk training too hard and you would not want to do that unless you are an experienced athlete in excellent health and fitness. Believe me, I've tried it the hard way and now I know that it doesn't pay off in the long run.
What should be the lower limit of your aerobic training zone then? Theoretically the optimal training zone should be as close to your max aerobic heart rate as possible, say within 10 beats or so. But if you believe that walking is a beneficial form of aerobic exercise, then your lower limit is whatever your walking heart rate happens to be. Happy training!