I used to be a total treadmill trotter settler. I'd get to the gym and plug in 6.5 mph and run for 45 minutes and get off and do that 3-4x per week with no change. I became bored to death and so did my body. I started feeling less fit after months of effort which was very frustrating.
The thing with exercise is you need to constantly be upping the anti with yourself in order for it to work properly. You can't coast and expect your body to continue to evolve, even if you're just jogging on the treadmill I've found that it makes a huge difference if you challenge yourself a little bit with each and every workout.
What I like to do now is to start my runs at a warm-up pace (for me that's 7.0 mph) then I add .5 mph every five minutes until I feel like I've hit my steady-state limit (usually around 9.0 mph) and then I hold that pace for the rest of the workout or if I'm tired and have pushed it a little too much I take it down .5 for 5 minutes like a ladder. Try this technique of upping your speed by .3 – .5 mph every five minutes in your next longer run and over time you'll notice a huge difference in your speed and running ability. My sprints used to be at what I run steady state workouts in now! The beautiful thing about running on a treadmill is you can push yourself and not have to worry about getting home! You can always take it down a notch if you go too far.
My causal jog pace is now a 8.5 – 9.0 mph for 40 minutes. I feel more comfortable running at that speed now than I ever did when I was just running at a steady state pace for miles and miles.
Bottom Line: You're not going to get fast (or more fit) by settling in! Try to up your treadmill speed each and every workout. Overtime you'll notice a huge change in your workouts and so will your body!