Why does the Torah insert the first "Mikem" before "Chamishah"?
Rashi: To teach us that even the weakest 1 among us will pursue the enemy.
From the word 'Mach' which means 'weak'. See also Ba'al ha'Turim.
If five will pursue a hundred of the enemy, then a hundred will pursue two thousand. Why does the Pasuk say ten thousand?
Rashi: Because one cannot compare many who learn Torah to few who learn Torah. 1
See Sifsei Chachamim. The question arises - What does learning Torah have to do with chasing the enemy? Why did Rashi not simply answer that one cannot compare many who chase the enemy to few who chase the enemy - which will also resolve the difficulty posed by the Sifsei Chachamim.
Having already written "Venaflu Lifneichem le'Charev" in the previous Pasuk, why does the Torah repeat it here?
Rashi: To teach us that they will fall before us in a most unconventional manner.
Ramban #1: The previous Pasuk is speaking on the battle-field, where they will pursue the enemy as they begin to flee and kill them by the sword, whereas the Pasuk now adds that after the enemy have fled the battle-field, Hashem will grant Yisrael the courage to give chase to far superior number, 1 and not only will they chase them, but the five will succeed in killing the hundred.
Ramban #2 (citing Ibn Ezra): It is an indication that they will fall time and time again.
Seforno: The previous Pasuk refers to Yisrael pursuing the enemy outside our territory, and the enemy falling miraculously at the Hand of Hashem. 2