The best thing about walking is it can be done anywhere at any time! Walking is one of the most accessible forms of cardiovascular exercise and the level of intensity can go from beginner to advanced.
If you are just starting out, make sure you walk a route that is relatively flat (if you're walking outside) or
As your fitness level increases, you can add hills or incline to your workout. For the more advanced walker, make sure you are really pumping your arms for an added benefit.