Why don't you like to run?
Walking will eventually lose its edge as a primary cardio workout, if its all you do. That, and your body will adapt specifically to that route. That's why multiple forms of cardio in multiple routes is important.
For example, I can walk pretty quick up an incline as steep as 20% (steepest near my house) with out any significant shortness of breath. Without breaking into even a light jog, I feel no significant effect from a walk.
Eventually, you'll need to upgrade, if you want to continue a gain.