How do you fix your grass in ACCF? I didn't know that when you run too much your grass goes away. HELP I'm Running and I can't stop!

Grass grows back with time, so the only solution would be time-traveling or just waiting. There is a good information page about it here, on the Animal Crossing City Wiki.

Also, just a tip for next time: Questions about AC should go on either the AC:CF General board or GG Animal Crossing board.

Thank you!

flowers help to.  if you fill your town with flowers your grass will grow back within a month or two.  if you plant gold roses everywhere then you won't have to worry about watering them. it is the only flower that doesn't have to be watered, ever.