As walking becomes easier, some people might choose to add in some jogging and then eventually run. This is definitely an intermediate to advanced level of exercise. Do not start out jogging or running if you have been sedentary or have any medical conditions that might be exasperated by this higher level of intensity.
Just like walking, you can vary the level of difficulty by moving from flat/level ground to incorporating hills and or increasing the incline on the treadmill. To add variety, you might want to try walking/running intervals.