Seconding CMW. Because the each leg jumps forward quickly and takes many frames to go back (=moves backward slowly), this looks more like a skating animation than a running one. Try taking the front leg in the 3rd and 6th frames and moving that into the previous frames, and flipping the legs in the 3rd and 4th frame (so that the leg going back goes back quicker, and so that the transition between the leg coming forward and it being forward isn't as sharp).
thanks for the feedback