I agree with Zorbs - just walking faster is the simplest way to add challenge to walking.
Next probably comes adding some hills to your route.
Finally, carry some additional weight in a backpack or weight vest (please no hand or ankle weights, as they stress the joints in ways they are not meant to be stressed).
Once you have maxed out all three of these, you may want to consider transitioning to running through a Couch to 5K program. These programs work through progressively increasing intervals of running and walking. www.sparkpeople.com/resource/fitness_artic
The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.
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