Here is an article talking about how running on a treadmill can affect your workouts outside. They mention that using the treadmill at a 3% incline will help you transition from inside to outside running.
Speaking from my own treadmill experience, I ran five to six miles a day at 0% incline. During this time, I was getting faster and my endurance was improving. However, one day my treadmill broke and I decided to try going outside. I went for a three mile long run during which I could not go as fast as I was used to. Upon returning to the treadmill I changed the incline to 1%, which greatly improved my running outside. This came in handy as my treadmill broke again and again and I found myself running outside for an entire month.
Do you use an incline on your treadmill? Do you find it difficult or easy to go from running inside to outside?