In this aliyah, Moshe Rabbeinu takes a break from his mussar to the Bnei Yisrael to perform a major mitzvah. Even though the three cities of refuge (arei miklat) on the eastern side of the Yarden would not actually begin protecting inadvertent killers until the other three cities in Eretz Yisrael proper were designated, Moshe does not wait. He sets them aside immediately, showing us his incredible love for mitzvos and how a Yid must run to do a mitzvah the very first second he gets the chance.
Following this, the Torah transitions back to the main teaching that Moshe is presenting to the Bnei Yisrael. The Torah details the specific geographical setting where Moshe is delivering these mitzvos, reminding the nation of the great miracles Hashem performed for them in defeating the mighty Amorite kings, Sihon and Og, and giving them possession of their lands.
The aliyah begins with Moshe Rabbeinu designating Bezer, Ramoth, and Golan as the three arei miklat on the eastern side of the Yarden for anyone who kills another Jew b'shogeg. The Torah then introduces the main body of the Torah and mitzvos that Moshe is setting before the Bnei Yisrael, describing the location of this teaching in the valley near Beis Peor. This is the very land of Sihon and Og that the Bnei Yisrael conquered under Moshe's leadership, stretching from Aroer on the Arnon valley all the way to Mount Hermon and the Arabah Sea.
The Torah tells us, "Az yavdil Moshe shalosh arim", then Moshe Rabbeinu set aside three cities of refuge on the east side of the Jordan. Rashi is immediately bothered by a major kasha: why did Moshe do this now? These three cities could not actually function to protect anyone until the other three cities inside Eretz Yisrael proper were set aside years later.
The teretz Rashi brings is beautiful and contains a tremendous chiddush for our own avodah. Moshe Rabbeinu knew he would never enter Eretz Yisrael, and therefore he would never be able to fulfill the mitzvah of setting up the final three cities. Yet, Moshe said to himself: "A mitzvah that is possible for me to begin, I will at least start!" Even though the mitzvah would remain incomplete and non-functional in his lifetime, Moshe did not say, "What's the point?" He ran with eagerness to do whatever small part of the ratzon Hashem was available to him right then.
Lichoira, we often fall into a dangerous trap of the yetzer hara where we think in terms of "all or nothing." We tell ourselves that if we cannot finish the entire masechta, there is no point in learning one amud. If we cannot daven with perfect kavanah from beginning to end, why bother trying to focus at all? Moshe Rabbeinu teaches us the ultimate yesod of a Yid's life: we do not work for the completion; we work for the ratzon of Hashem. If Hashem gives you an opportunity to do even a fraction of a mitzvah, you grab it with both hands.
Today, do not let the "all or nothing" mindset hold you back from your avodas Hashem. If you only have five minutes to open a sefer, or if you can only say one brachah with real focus, do it anyway. Grab that piece of the mitzvah and run with it, just like Moshe Rabbeinu.