Very good animation. But there are some few flaw errors.
1. The back seemed a bit akward the way it moved back and forth but not bending. when the runner jumps into the air have his back straighten a bit, then when he hits the ground make him bend it a bit.
2. The head didn't seemed like it moved at all. Too make the run look realistic Make the head bend with the body like when you jump into the air make it move upwards, when he hits the ground make it move down a bit. Make sure thats its only a little otherwise it will look un realistic.
If you need an example
hereGreat animation though, and you will get better in no time.