For the use of the imperfect here, see Smyth 1908:http://www.perseus.tufts.edu/hopper/text?doc=Smyth+grammar+1908&fromdoc=Perseus%3Atext%3A1999.04.0007
ἐγὼ μὲν ἐξώρμων ἕωθεν. ᾗ δ' ἐγὼ
ἀπῇρον ἡμέρᾳ, λαθόντες τοὺς σκοποὺς
τοὺς ἡμετέρους, οἱ βάρβαροι, λόφον τινα
ἐπίπροσθ' ἔχοντες, ἔμενον, αὐτομόλων τινῶν
πεπυσμένοι τὴν δύναμιν ἐσκεδασμένην.
ὡς δ' ἐγένεθ' ἑσπέρα, κατὰ σκηνάς θ' ἅπαν
ἦν τὸ στρατόπεδον, ἔκ τε χώρας ἄφθονα
ἅπαντ' ἐχούσης, οἷον εἰκὸς γίνεται,
ἐβρύαζον οἱ πλεῖστοι . . .
men/de sets up a contrast between two things going on the same time. Daos men
was setting out at dawn; on the very same day de
that he was setting sail, the barbarians were [staying] on the hill-top which they had seized (aorist particple). As Smyth notes, the imperfect puts the listener in the midst of the events as they were taking place. It's a more vivid, breathless way of telling the story.
I don't think you should twist yourself in knots trying to make real Greek conform to the black-and-white grammar book rule that the aorist denotes completed actions while the imperfect denotes actions that are in process. Verbal aspects is a very subtle feature of all languages--at least all that I've studied. Ancient Greek sees things somewhat differently from English; the Greek aorist doesn't always map perfectly onto the simple English preterite and the imperfect doesn't always map perfectly onto the English progressive past. The meaning here is clear--I would suggest that you just notice how the Greek expresses it and note the difference from English.
Note: I went back and fixed the slip noted by mwh below. Also, I punctuated the passage, using periods (full stops) where normal Greek punctuation would use raised dots (semi-colons), which my keyboard doesn't allow me to use. I wanted to present the passage as verse, which makes it much easier to follow the syntax, or lack thereof (and to supply a critical word or two Daivid left out).