Hi freshman15! When you say "significantly easier," I assume you are referring to the amount of effort expended on the various surfaces. One would assume that even the flattest of outdoor roads isn't as consistently flat as the track you're running on indoors. With that being said, the roads on which I run are pretty level, so there's not as big a difference in track-running versus road-running for me. You might try using this "HIIT" (high-intensity interval training) everyone is talking about these days, a description of which can be found in the exercise forum here. Essentially, switching up your pace during your run, as opposed to running at a constant speed, is a great means of upping your intensity.
Another idea: do you do any weight training? If you work out in a gym where the treadmill is near the weights, try running full speed for a certain distance, then immediately moving over to do a set of weights, all the while keeping your heart rate up (while not sacrificing form). You know, like circuit training. Then hop back on the treadmill, etc. My trainer has me do this and I am worn out at the end of my session. Of course, the trick is to ensure your gym isn't so crowded as to have your treadmill stolen while you're gone for 90 seconds...