Why does it say "Lo Yetzar Tza'adecha"?
Rashi: One who does not distance his feet [while walking] is prone to fall.
R. Yonah: "U'Mah Helesicha" (Michah 6:3), "Ki Yagata Bi" (Yeshayah 43:22). The Avodah is not toil - if you toiled the entire day for an optional matter. One is in the market the entire day and does not weary. You weary because you serve Hashem - "Karov Atah b'Fihem v'Rachok mi'Kilyoseihem" (Yirmeyah 12:2).
Malbim: Sometimes one goes in the way of the Torah simply - he fulfills Mitzvos according to law and lest he be negligent. This is "b'Lechtecha". Then, his steps must not be short, i.e. to be lazy in Mitzvos. Rather, he does them properly and goes in the regular path.
Why does it add "Lo Sikashel"?
R. Yonah: You need not be concerned lest you find a stumbling block in the straight path of Avodah, like one who runs - he fears a stumbling block on the road. Those who go to do a Mitzvah are not harmed (Pesachim 8a), "Lo Yar'iv Hashem Nefesh Tzadik" (10:3).
Malbim: Sometimes one runs in the way of Torah - he does Midas Chasidus and runs with fervor to add fences, Kedushah and Perishus. Then he must beware of stumbling, like the four who entered the Pardes (Tosefta Chagigah 2:3); one of them cut saplings (profaned Kedushah). About this, it says "you will run and not stumble."
