As with most improvement techniques, some habits will have to be changed. However, you will likely feel the improvement immediately. The energy spent here has no value in running. A good way to tell if your face is relaxed is to do a cheek-check. If your face is relaxed while running, you will feel a little bounce in your cheeks.
Shoulders are another culprit.
Hunching up those babies is hard work with no return for your effort. Concentrate on relaxing the shoulder area, particularly in the later stages of the workout. Arms should be carried low, moving back and forth to keep your body going straight. Arm curls can be very impressive, but save 'em for the beach. Top runners often relax in mid-race by dropping their arms down to their sides.