Answer:
I can be written, I can be spoken, I can be exposed, I can be broken.
The more you take, the more you leave behind.
What measures out time. Until in time all is smashed to it?
Walk on the living, they don't even mumble. Walk on the dead, they mutter and grumble.
I appear in the morning. But am always there. You can never see me. Though I am everywhere. By night I am gone, though I sometimes never was. Nothing can defeat me. But I am easily gone.
I run through hills. I veer around mountains. I leap over rivers. And crawl through the forests. Step out your door to find me.