Did I hear.........."trailrunner"?

Get a pack designed for trail running. Those ultralight sacks with 2 strings for your shoulders and no hip belt have their place, but you want a pack with lots of straps to hold it to your body. Otherwise the bouncing will drive you crazy. At least it did for me. Look at adventure racing packs.

If you're going overnight, forget a stove or a tent. "Bivy sack" is a loose term that can be anything from a few ounces to a few pounds. You need to be on the lower end which means little more than bag cover.

I have done this in the mountains where water is everywhere. It might be more tricky in the desert. If it's hot you'll need lots of H20, which should be your heaviest single item.