Living in the Valley of Hills
I live in an area with a LOT of hills. In fact, that's the reason I postpone learning how to drive a manual car. I seem to roll backwards too often, and quite frankly, it scares me to death. Not only do hills affect driving, but also running.
Hills are good for running. I once read somewhere that it's good to throw in a hill every now and again to your run. Living where I do, that's not really a problem. I found another source of information with benefits of running hills.
After reading the article, I am motivated to really run as many hills as I can. The article mentions benefits with "training distances, anaerobic capacity, and strength." With my own experience, pushing yourself beyond a typical run (including adding hills) does a lot more than benefit you physically. The mental state to push yourself while running also adds unmeasurable benefits. It's always fun to come home from a run where you know you pushed yourself a little more than your last run and prove to yourself, one more time, you can do anything you put your mind to.