StuffedBunny - I increase/decrease the speed while on the treadmill and speed up as needed. I started at 5.0mph and just increased my speed by 0.1mph at a time until it felt "comfortable". I think that's my problem with treadmills. I'm stuck going x mph the whole time whereas I slow down slightly naturally when running outside when I need to and I think that helps me to recover faster and run longer/farther.
Back on topic... When I went from a treadmill to outside (aka the weather got marginally better) I didn't feel any more like a runner. Just relief that running outside was easier than everyone I knew made it out to be
ETA: I use an incline of 1% now (that's what's recommended to make the treadmill feel like "outside") but I started at 0 and worked up.