From the heights of Mt Pentele the beacon flared, telling Athens that enemy forces had landed.
From the moment that the fall of Eretria became known, a succession of fierce debates had taken place in the Athenian Assembly. Some were for sitting tight and holding the city against a siege, that was sure to come. Others, Miltiades in particular, insisted that the citizen-army should go out and fight. A siege would cut Athens off from any type of help that the Spartans could offer. Spartan help would not be able to get into the city before engaging the Persians. The risk of treachery within the city had to be considered as well. There was a pro-Persian faction inside Athens, and Eretria was finally won over this way, the possiblity that it could happen to Athens was all too real.
The Athenians sent a herald Pheidippides to Sparta. He left Athens while it was still dark, and reached Sparta by the following evening, having covered something like 140 miles over bad mountainous roads  Upon his arrival, by the 5th of August, he spoke to the rulers and pleaded with them for assistance for Athens who with the Persians on their very doorstep had come to enslave them, already they had taken Eretria.
The answer he got back was not pleasing to hear, the Spartans were full of sympathy, but regretted that they could not put troops into the field until after the full moon- that is, on the 11th or 12th of August. To do so would mean breaking a religious taboo, probably in connection with the Carneian Festival, sacred to Apollo. A force was in the readiness at the frontier, prepared to move as the moon, or the luck of battle, dictated. By the time this force would enter into Attica and be in a position to threaten the invaders it would be another ten days. 
Pheidippides headed back to Athens with the grim news from Sparta. On his way back home he must of wondered, if he even had a city to go back to. While on his way when he was around the city of Teaga he claims to have run into the god Pan. 
Modern historians have always passed over the meeting between Pheidippides and the god Pan, as a hallucination due to lack of sleep , exhaustion etc, and continued going on with the story. However, there is a more sinister explanation if one delves a little more under the surface [See Pheidippides mets Pan]