I've had great results using the treadmill for stretches at a time. For whatever reason though I have to go by effort and am almost always .8 to a full 1.0 mph slower on the treadmill than what I run outside, while at an incline of 1.0 for the easy paces, and 2.0 for the faster work.
Not only is the effort/pace a little different for me, it is far less linear the slower I go on a treadmill. 3-4 mph at 2.0 incline, I swear feels more like a 10 min/mi pace for me. (Perhaps there is something to what I have read, being that physical therapist Shirley Sahrmann was noted as saying to avoid running faster than 4.0 mph on a treadmill as the pattern for keeping up becomes incorrect).
Also, regarding effort. monitoring my effort by HR has also backed up what I feel regarding running slower than I do as compared to outdoors, and after long stretches of almost exclusively treadmill running (3 weeks+) have found myself able to pick up right where I left off outside.
In my area there is a local masters runner, lower volume of 30-35 miles per week, and ran mid-18s who did a lot of treadmill running. If I recall correctly, he ran the same paces on the treadmill as he did outdoors, but he would go up to a 1.5 to 2.0 incline for easy pace work, and then on his interval or tempo work would gradually go from 2.0 to 4.0 throughout the workout. One of the most consistent racers I've ever seen throughout a race as a whole in terms of pacing throughout.