Carl Friedrich Keil and Franz Delitzsch Old Testament Commentary

As Ahimaaz still expressed a wish to hasten to the king, even after Cushi had been sent, and could not be induced to relinquish his purpose by the repeated expostulations of Joab, the latter at length permitted him to run. And he ran so fast, that he got before Cushi. מה ויהי: let whatever will happen. וּלכה is the pronoun "to thee," as in Gen 27:37, and not the imperative of הלך, "thou mayest go." The meaning is, "and there is no striking message for thee," no message that strikes the mark, or affects anything. We must supply "he said" in thought before Sa2 18:23. There was the less necessity to write it here (as in Sa1 1:20), since it is perfectly obvious from the repetition of מה ויהי that it is Ahimaaz who is speaking. Ahimaaz then ran by the way of the plain, i.e., the way which lies through or across the plain of the Jordan. Now he could not possibly have taken this road, if the battle had been fought in a wood on the eastern side of the Jordan, and he had wanted to hurry from the scene of battle to Mahanaim; for in that case he would have taken a circuitous route two or three times the distance of the straight road, so that it would have been utterly impossible for him to get there before the Cushite, however quickly he might run. This notice therefore furnishes a decisive proof that the battle was fought upon the mountains of Ephraim, in the land to the west of the Jordan, since the straight road thence to Mahanaim would lie through the valley of the Jordan.