I'm lucky, I live very close to a large park where I can run around a lake and in (carefully maintained, totally artificial) forests, I also have a treadmill which was being sold on craigslist by someone in my neighborhood who'd barely used it.
I get a better workout on the treadmill.
Frequently I hear people say they can't run on a treadmill, it is too boring, and at times I've thought so too.. but consistently I found I could run further and longer on the treadmill and I always put the treadmill at an incline to compensate but still keep running longer and faster.
Finally this weekend, as I was walking along the lake halfway through my run, I figured it out. The monotonous boredom thing is actually useful to me. My best runs are when running becomes meditative, its just me and the motion. When outdoors I'm constantly distracted, constantly finding reasons to stop and smell the flowers, and I have a great time but hardly a run. Outdoors I find I have to constantly remind myself to keep moving, its a forceful effort. Indoors on the treadmill its stopping that requires a more conscious thought because I have to reach out and press stop before I can let my legs halt. When I don't have to think "keep running, keep running, don't stop" through my head to move each step its easier for me to just revel in the movements themselves.
This does not mean I'm giving up on running outside, its great for my mood so long as I don't beat myself up afterward about walking half of it and spending 5 minutes staring at a dog or the 5 couples getting married or a flock of geese. Instead I'll continue to put on my running gear on the rare occasion that I have daylight to run in and run as I feel like and walk as I wish.