“ἐνταῦθα ἔμεινεν ὁ Κῦρος καὶ ἡ στρατιὰ ἡμέρας εἴκοσιν· οἱ γὰρ στρατιῶται οὐκ ἔφασαν ἰέναι τοῦ πρόσω· ὑπώπτευον
γὰρ ἤδη ἐπὶ βασιλέα ἰέναι·”
Steadman refers to this as the genitive of place where, but I have no idea what that even is, or how it fits in with προσω,what difference it would make if it didn’t exist etc.
The Cambridge lexicon lists it as an alternative to πρόσω without the τοῦ, so it wouldn’t, presumably, affect the meaning if it were absent. Goodwin 1138 mentions this as a genitive of place which occurs in “a few expressions” in Attic prose, and that “these genitives are variously explained” but doesn’t say more than thst
this sentence is quoted in Smythe 1448. The article is substantivising the adverb.