וְעָשָׂ֤ה יְהוָה֙ לָהֶ֔ם כַּאֲשֶׁ֣ר עָשָׂ֗ה לְסִיח֥וֹן וּלְע֛וֹג מַלְכֵ֥י הָאֱמֹרִ֖י וּלְאַרְצָ֑ם אֲשֶׁ֥ר הִשְׁמִ֖יד אֹתָֽם׃
Y hará SEÑOR con ellos como hizo con Sehón y con Og, reyes de los Amorrheos, y con su tierra, que los destruyó.
Ibn Ezra
To go to and fro denotes going out to, and returning from, battle. In other words: “Even if I were not now going to die, still I am incapable of waging war. Thus, you have no need of someone who would leave you unattended. But rather, God will destroy the nations, and Joshua also — and the proof is that which you have seen with your very eyes, namely, the war of Siḥon and ‘Og.”