Render: And now the Amalekites and the Canaanites are dwelling (or abiding) in the valley: wherefore turn you, etc. (that so ye be not smitten before them). The Amalekites were the nomad bands that roved through the open pastures of the plain Numbers 14:45 : the Canaanites, a term here taken in its wider sense, were the Amorites of the neighboring cities (compare Numbers 14:45 with Deuteronomy 1:44), who probably lived in league with the Amalekites.
Tomorrow - Not necessarily the next day, but an idiom for “hereafter,” “henceforward” (compare the marginal reading in Exodus 13:14; Joshua 4:6).
By the way of the Red sea - That is, apparently, by the eastern or Elanitic gulf.