The initial story that was told just involved the run to Sparta in a single day prior to the battle to ask for assistance. After the Athenians won the battle, they marched the 40km to Athens in full armor in order to arrive before the retreating Persians could launch a naval attack on the city itself. Later, these two stories got combined into the popular legend that’s described in this post.

So yeah, the rush was that they were trying to outpace an incoming invasion. They won the battle, but the war was still on. Their rush back to Athens allowed them to stop a direct assault on the city, which led the Persians to retreat entirely.

You are expecting too much from Trump. His pattern is to proclaim some new policy with shaky justification and no prior analysis. At best, his thought pattern is, “20% sounds like a good number.” Then he lets the rest of his administration try to define something of substance around his public statements, hoping that he doesn’t contradict their attempts in the coming days.