the best way i can describe the feeling i get is when you think there's a step down to something and there isn't actually a step and you kinda fall forward awkwardly?
forgive my horrid terminology, it's been a while since i've really talked with anyone about animating so i've forgotten my lingo, haha, but the "extension" phase of a run looks more like this:

legs waaay outstretched, knees up. this guy is going for a leisurely sprint apparently (its from an old school project of mine, don't laugh haha,) obviously leg movement varies with speed; but basically your guy's legs are at a jog but his upper body gotta go fast. the head movement is kinda driving the feeling that he stepped off the curb wrong, though if he had a bit more extended of a stride it'd be fine i think. over-exaggeration is never bad when animating
also, on his extention, he doesn't really lift up any. You kind hop up when you run so your head actually bobs quite a bit. here's an example from my book because this guy shows it better than i can explain and i dont wanna draw this myself:

this is a walk but you see what i mean. it'd be a greater curve during a run (he does illustrate this as a run but not as plainly.) this usually, with traditional, be accomplished just by moving your frame up a bit in post but since you drew the line for him to run on i thought i'd point that out!
man it's been too long since i've done any proper run/walks (my current project is a retro game so its all very low-frames, weh) and now i really want to but it's pretty much midnight :'[