Treadmill is typically a weight bearing exercise. This means the heavier you are, the more you are going to work.
As for an incline, a 20-30 degree incline at 3-4 mph, for 20-30 minutes will be far better than walking on a flat surface. I'm sure you notice that as you walk longer, it becomes harder to maintain. I say keep at it, and be sure you keep your heart rate at the training level.
However, you should note that inclined treadmills are not exactly ideal for running. Sure it reduced impact to your knees by a lot, but you can get far more benefits by going outside and running on the proper surface. I highly recommend finding a track near you and running on the track. I'd stay away from street running/walking unless you want to kill your knees. Typical streets are sloped downwards as to keep the water / oil off the roads. Fine for cars, but when a person is running on it, it puts more pressure on one leg than the other. Fine ingredients to screw up your knees for life.