Albert Barnes Commentary 2 Samuel 18:23

"But come what may, [said he], I will run. And he said unto him, Run. Then Ahimaaz ran by the way of the Plain, and outran the Cushite." — 2 Samuel 18:23 (ASV)

The plain - The floor of the valley through which the Jordan runs. The Cushite did not run by that road, but took the road over the hills, which may well have been the shorter but also the more difficult road. The two roads would probably meet a short distance from Mahanaim.

These words, which have been thought to prove that the battle took place on the west of the Jordan, are clear proof that it took place on the east. This is because if the runners had to cross the Jordan, they both must have taken the same road, which it is clear they did not.