He seems kind of over-bulky for a ninja. I tend to think of ninjas as lithe and nimble. Feels like bulk would kind of slow him down. It also doesn't really come across as a very "ninja-ish" sort of run. You could get a lot more characterisation by giving him perhaps a lower, more carefully silent run, like he's trying to remain stealth. The outfit also seems kind of strange; it looks like he's at the beach or something. Unless this is just a kind of base or something?
But addressing the animation as it is, I think you should really block in the entire figure's motion first, and then work on the rendering. The lack of arms kind of throws off any solid idea of how the torso should be moving. I get a feeling like the torso isn't twisting enough, but I find it difficult to tell.
The way the legs snap forward just before contact with the ground (frame 1 to frame 2) doesn't work well, I think. It's giving the motion a very unnatural kind of flick, like he's trying as hard as he can to kick his leg out infront of him, and then smacking his heel into the ground.
There's a volume distortion in the legs when they reach fully behind him. They stretch out for a moment, so it loses any sense of the mechanics of it swinging from the knee.