"An old horse rides away from the dust, aiming at a thousand miles" is a metaphor for people who are ambitious, but they are still ambitious when they are old. Poems similar to "an old horse rides high and aims at a thousand miles", such as: the sky is high, the sea is wide and the fish jumps: if you are ambitious, how much you may achieve, of course, you still have to rely on yourself.
Therefore, the answer is: the sky is high for birds to fly and the sea is wide for fish to jump.