If you have been doing the same workout for a while it could be your body has adjusted and you need to shake things up a little bit. This article can help you add intervals to your walks to increase the intensity that may help.
Walking Workouts with Intervals
Training Programs for Beginner, Intermediate, and Advanced Walkers www.sparkpeople.com/resource/fitness_artic
Here is more about the benefit of interval training...
Get Results with Interval Training
Add Fun and Variety to Your Workouts www.sparkpeople.com/resource/fitness_artic