Chasing the sun- no matter how long you head into the sun it will always be there on the horizon out of reach.
A rainbow is another thing. You can never get the pot of gold at the end.
Tomorrow. Tomorrow is always out of reach because as soon as you get to "tomorrow" it becomes today, so you never really reach tomorrow.
I'll edit this if I think of anything else.
EDIT: What are you using this metaphor in reference to?
Like a weak hungry lioness at the heels of a springbok (or some other type of prey) that is just too quick.
To run on water
Escaping from a black hole.
Drinking all the water in the ocean.
Surviving with no blood at all in your body.
It was a receding light at the end of the tunnel.
It was far away, a mirage in the desert just out of my grasp.
It was a cookie jar on the top shelf.
chasing the horizon
Just like chasing light.
Doubt those are good, but maybe you could use one.
it was receding into the distance and each time i attempted a snatch at it, it would escape, barely missing my desperate fingertips