Thanks for the great advice guys.
Mjigga99, I think I'm generally in good physical condition, so I don't think it's an issue of being out of shape.
Edlo, that's a lot of good stuff you put down. The feeling that I get is like when I'm standing straight up on my toes without a board, and when I'm actually strapped onto a board, I feel that the fatigue feeling is even worse, so I guess I'm doing it wrong.
You also bring up a great point. I'm always trying to control my speed by skidding or trying to stop completely, and most of my muscle fatigue comes from trying to skid/stop with the toe edge. So you're right, maybe I'm having to fight gravity too much as I try to go perpendicular to the hill. My heel edge is better and I don't usually get tired too much from that.
Snowklinger, that's good advice. I'm going to try to use my shin on the toe edge to control the board, rather than using my feet exclusively. That should cut down on the fatigue too.