I simplified it to 4 frames for the sake of simplicity.
If you get rid of your "tween" frames then step through the motion and observe one leg at a time then the other, you'll notice both leg motions are essentially backwards. I swapped frames 2 and 4, now they are stepping in a forward motion. I also made his arms swing back and forth in a pendulum motion rather than a circular motion to mimic how I would run in real life.