Contrary to popular belief, Marathon was not a major battle, not in the magnitude of Salamis anyway.

The Persians were trying to create a buffer between the Greek cities of West Asia Minor and the mainland, especially the aggressive and supportive of rebellions Athens. This was to be achieved by placing Ippias as a tyrant in Athens. The first leg was successful. Naxos, the largest island of the Cyclades was occupied and the Eretrians defeated (showing that the Great King exacted reverge on his enemies). The second part of the operation however was not successful.

If it was successful, then the Persians may have managed to place Ippias as a tyrant in Athens. I say may, because tyranny was not very popular in Athens. Sure there were supporters but in the end the democrats would have prevailed. What would happen after that is pure speculations. Perhaps history would have followed approximately the same path, but with a small delay. Perhaps, Xerxes would still attack Greece and win because the Athenians did not manage to build their fleet. Perhaps the Athenians would recover in time and manage to defeat Xerxes. Who knows.