I haven't animated for a sec so i did something simple.
http://sta.sh/01tcsf2qejv8
Like always CNC is appreciated.

I think that back leg might need to move backwards a bit more. the movements you might want to try exaggerating your movements a bit more.

I'd say the leg thats moving forword needs to extend out, instead of staying bent, it helps some times to just run in real life or watch some one else run

Try Giving it some bounce on each step to show how its a real run loop

If he's running his body should be bent further forward. His body should also move up and down slightly with each stride.