In what way can he not stand?
Radak: He cannot stand against the enemy, for he will not have strength to draw his bow.
Malbim: Sometimes archers stand and stop the enemy a little, and this enables the fast to flee. Now the archer will not stand at all; he will flee immediately.
What do we learn from "v'Kal b'Raglav Lo Yemalet"?
Rashi: The fast person will not save his life. Radak ? this is like the previous verse said "v'Avad Manos mi'Kal"; the matter is repeated to strengthen it.
Malbim: Because the archers will not impede the enemy at all, the fast person will not have time to save himself.
Why will a horse rider not be saved?
Radak: "Sheker ha'Sus li'Sshu'ah" (Tehilim 33:17), when Hashem does not want [to save].
