Bearing in mind that this Parshah was said on Rosh Chodesh, how can the Torah write that Yisrael carried out what Moshe and Aharon told them?
Rashi: Since Yisrael undertook to bring the Pesach, the Torah reckons as if they had already performed it.
Why does the Torah insert the word "va'Yelchu"?
Rashi: To teach us that they received reward for going to perform the Mitzvah as well as for the actual performance itself. 1
See Pirkei Avos, 5:14.
What is the significance of the phrase "Ka'asher Tzivah Hashem Es Moshe ve'Aharon"?
Rashi and Ramban (citing the Midrash): It teaches us that they performed what Moshe and Aharon told them, down to the last detail.
And why does the Torah add the words "Ken Asu"?
Rashi and Ramban #2 (citing the Midrash): Because Moshe and Aharon too, performed everything that Hashem instructed them down to the last detail. 1
Ramban #1: It is the way of the Torah to repeat that they did it, 2 to teach us that they performed what they had been told to perfection.
Ba'al ha'Turim: The two Asiyos (actions) refer to Milah and Pesach.
Da'as Chasam Sofer (Avodas Hashem 10): First Yisrael did, and then Moshe and Aharon. This shows that when going to do a Mitzvah, we do not honor Gedolim to let them do first. Whoever is zealous to do first, he gains!