2. A Great Trunk (Core)
Your trunk musculature provides stiffness so that muscle propulsion forces have something to push against. The earth is so massive compared to you, that it has no problem pushing back. If your trunk is too soft at initial impact of your foot, you'll dissipate more of the propulsion force than if your trunk is firmer. (btw, my main beef with too soft shoes:))
Some of my recommendations for working your trunk.
1. Planks. Prone, side, feet on an unstable surface or not. Tried and true.
2. Ball slams, med ball rotation throws against a wall. Satisfying too!
3. Suspension trainer. TRX is the icon here. I do like their adjusting buckles. Not sure it's worth the price of admission, but somebody must support their marketing machine!
4. Barefoot running. Hard to run poorly with your shoes off. You'll probably notice some muscles that are sore that you thought were strong. Start with a 1/4 mile at the end of a run. Add a 1/4 mile every week. If your surface is rough, wear minimalist shoes.
Happy Running!