This is about the best damn core exercise in the world.. maybe even in the whole county. This works some of the key muscles in the pelvis and upper leg that help you to maintain good (and efficient) run form. Once your core fatigues in a race or training then you lose the ability to move efficiently, this adds fatigue on top of fatigue. This exercise should be performed 2-3 times per week with the simple idea of building very gradually to a high volume of sets and repetitions. Start at a conservative 2 sets of 6-8 reps. and avoid soreness for the first few times. Then add first 1 set, then add repetitions as you feel on a lift to lift basis. Don't over think this.
I am also including a video of my ugly mug and my beautiful baby boy. I love this video.