Hey RIF REVers!
I've coached so many runners who initiate movement using their bodies splendidly. However, when asked to speed up, their technique returns to square 1: pumping the arms, relying on power and over-striding which straight-away causes them to work harder and put their body's health at risk.
The key is learning to speed up whilst not working harder, so you can maintain a low aerobic threshold, even moving at speed.
When running I teach beginner runners to focus on LRLS (Lean, Relax, Lengthen and Shoulders (down)). But when asked to increase speed those techniques fly out of the window. Runners that run distances with good speed need to maintain good alignment without over-working physiologically, to prevent against a break-down physically and from "hitting the wall."
One important aspect to wrap your head around is speed is created by movement BEHIND THE HIPS NOT IN FRONT OF THE HIPS. I say running technique is like a Mullet - business in the front, party in the back. So lengthening the upper-body, through the hips and the chest, creates movement with good posture and slight forwards lean but it's the movement behind the hips the create speed.
To move faster all that you need to do is lean slightly further from the ankles (remaining straight from head to toe!) The other CRUCIAL ELEMENTS to focus on are ensuring your feet and thighs are completely relaxed, that contact (of the feet) occurs below the hips and as your speed increases using this concept, ground forces (friction) increases so your feet move higher behind your body - almost kicking your bottom! So, with an increase in speed the movement in front of the hips remains minimal with the knee moving forwards (not lifting) and simply the feet are following through to optimise efficiently (in the same way you follow-through with a golf swing for increased accuracy).