It's nearly impossible to run forever with a basic run, you will end up falling over.

However, I noticed your abs were contracted the entire time. Relaxing them after the initiatial 20 frames should help. Also, when you start to lose balance, remember to use your lumbar.

I decided to mess around a bit, this is what I came up with. It's probably possible to run farther with the same move, I'm not the greatest at running.

Good luck!