He was probably not 60 at Marathon. The general convention when the birthdate of a figure from antiquity is unknown (particularly a Greek) is to take the first attested contemporary mention of his name, or if he's a writer or artist his first major work, and assume he was forty at the time. It's very often not correct or highly contested--Richmond Lattimore, for example, thought the traditional date of Aeschylus' birth (525) is nonsense, because it's forty years before the staging of the Persians, and Lattimore thought that Aeschylus and his brother would be too old to serve as notably at Marathon as we are told. Miltiades' birthdate is calculated from a cup that bears his name, dated sometime between 520-510. We have no idea whether this is the Miltiades (a number of members of his family bore the name) or how old he was at the time the cup was made. We use this for some reason, rather than his archonship in 524--I'm not sure why, I presume it's because it would make him 74 at Marathon, and that's too old. In any case, two things must be understood. First of all, Athenian citizens were expected to serve until they were quite old. Tyrtaeus mentions that it is a noble thing for an old man to serve in the front lines, describing a white-haired hoplite who dies rather gruesomely (he gets stabbed in the nuts) in the front rank, instead of a younger man. The other thing is that Miltiades was probably not as responsible for Marathon as tradition makes out. Herodotus claims that Miltiades decided to fight at Marathon, that he persuaded Callimachus to cast the deciding vote, and that the day of the attack was his decision. It's only modern scholars who actually put him in command of the Athenian army. The OCD notes that this is probably an anachronism--by Herodotus' time the polemarch served very little function, but during Marathon the polemarch was probably still commander-in-chief, and the ultimate decision would've been his, as would ultimate command over the army. Miltiades was undoubtedly important--from what Herodotus says he was influential in persuading the Athenians to fight a decisive action, even if the attributions of modern scholars are discarded. But he may not have been quite as important as modern scholarship would like to believe--the contributors to the article on Miltiades in the OCD suggest that his role as commander may have been inflated by Callimachus' death and the immense political influence of Cimon, who was Miltiades' son. But I think it's hardly unbelievable to suppose that Miltiades took his place in the ranks--plenty of other generals fought in the ranks despite old age (I mean, hell, at Ipsus Antigonus was 81, Lysimachus was 60, and Seleucus was fifty-something--yet they all fought among their troops, and Antigonus was killed in battle)