If your main form of cardio is running, it's important to mix up both the distance and pace you run at. FitSugar reader cmd0610 also reminded me that another thing you want to switch up is the surface you run on.
Since different surfaces are more forgiving than others, they offer different challenges to our muscles. If you can help it, I don't advise running often on rigid and stiff concrete because it offers no shock absorption to your joints, but I do recommend alternating between softer surfaces such as grass, dirt trails, a spongy track, treadmill, even asphalt (on the street, not on the sidewalk). Also alternate between flat and bumpy surfaces, and those that are uphill or downhill because they'll challenge your coordination, and help you become a more efficient runner.