Simplified Herodotus, as you said. Also with a few mistakes thrown in. Corrected version:
οἱ δὲ Ἀθηναίοι ἠσαν ἐν μεγάλῳ κινδύνῳ; ὁ οὐν Φιλιππίδης ἐπέμφθη κῆρυξ εἰς Σπάρτην ὑπὸ τῶν στρατηγῶν
The Athenians were in great danger. Then Pheidippides was sent as a herald to Sparta by the generals. [The order of words as in the Greek.]