Treadmill running can reach an even plateau by making changes to the way that you use the treadmill to achieve your goals. You should first realize that running on the treadmill with 0% incline your exertion is less than running outdoors because the belt helps to bring your foot backwards. The way to incorporate that into your run is to increase the incline to 1% to make up for the decreased exertion.
Next find something to use as a distraction. Many people find that they can run further outside than they can on the treadmill because the perceived exertion is greater on the treadmill. Researchers have found that runners who use a distraction, such as music, television or listening to books on tape, can provide enough of a distraction to keep your mind off of any perceived pain or discomfort.