Action!
I always like it when and artist tries to display action in a piece of artwork.
I'm in agreement with Zephyrduo about the feet, but I would also like to point out the shirt and arm that seems to be meant as a foreground object to help with the perspective.
While running the shirt should be pressed more to the front of the body, so there should be more deffinition of the front of the body rather than fullness that is present currently. Even with the up and downward bounce that is present in the clothing while running, you are presenting up with a moment in time.
The arm on the right throws me off since the shadow and angle of the upper arm is more in tune with the arms falling behind her such as the other on the left. Since it is in front of the body, the hand and lower arm should also be larger, were as it is now... only the hand is.
I would like to help offer you a red line, or a tutorial example if you would like that. In the mean time, keep up the good practice and hard work! You can only improve from here on!