R. David Kamchi explains the word choice as follows (I'm taking the first part from here):
as rendered by Onkelos, "serve Me" by carrying out what I command you ( in order to become perfect). This is also that meaning of the word התהלך in Genesis 24,40 as well as in Genesis 48,15. It is a reference to serving the lord in thought and deed. All service of the Lord is rooted in man's heart. This is why King Chiskiyah in Kings II 20,3 in his prayer says: אני ה' דבר נא את אשר התהלכתי לפניך באמת ובלבב שלם, "please o Lord remember how I have served you in truth and with a pure heart, etc." Chiskiyah first refers to his serving G'd with his heart, before mentioning that he had performed deeds which constituted service of the Lord also, when he said והטוב בעיניך עשיתי, "and I have done what is pleasing in Your eyes."
The Shadal speaks of the word choice as referring to a more physical sense of being guided.
התהלך לפני: כתינוק המהלך לפני אביו שאביו הולך אחריו ומנהל אותו
The Tur HaAroch refers to the Ramban and writes
Nachmanides explains the word התהלך as similar to the commandment to the Jewish people in Deut. 13,5 אחרי ה' אלוקיך תלכו, i.e. “follow all the instructions of the Lord your G’d.” (after you have been instructed about them) The command התהלך לפני goes beyond this and asks Avraham to anticipate G’d’s commandments and to act in accordance with his understanding of what G’d would later on command him. [it is difficult to imagine a greater compliment to Avraham than our verse here. Ed.]