It REALLY depends on you. For me, running at 4-4.5 MPH feels bizarre; that's a walking speed for me. However, my friend who is much shorter is running comfortably there. For me to actually run, I need to be at around 5 MPH to be comfortable.
Experiment with what works for you. What matters isn't what speed you're going, but that you're running (both feet leaving the ground at one time.) Your speed may be painfully slow for someone else, but that doesn't mean that you aren't getting a workout.