Why will he run in "Derech Mitzvosecha"?
Radak: You will widen his heart so much that he will able to run in them without hindrance.
Malbim: Derech is the big path; Nesiv is for individuals. Even though he asks afterwards that Hashem guide him in "Nesiv Mitzvosecha" (35), he will run in Derech ha'Mitzvos. Indeed, the details are hidden from him - "Al Taster Mimeni Mitzvosecha" (19). However, the big path leading to the Mitzvos, which are to arouse Emunah, good Midos or obligations Bein Adam l'Chavero, I know it and run quickly in it.
Why does he say "widen my heart"?
Malbim: The heart is the power that rules [over man]. "Derech Mitzvosecha" will widen it. Sometimes it is in straits due to enemies of the Nefesh that torment it, i.e. lust, anger, envy and similar [Midos]. The Derech will widen his heart over all its afflictions. In it, he will defeat the enemies of his Nefesh.
