How will the mountains become a path?
Rashi: Via its destruction, "Shavas Over Orach" (33:8); now, passersby will resume going there.
Radak #1: This is like it says, "Kol Ge Yinasei v'Chol Har v'Giv'ah Yishpalu" (40:4). This miracle will evade the need to ascend or descend on the road.
Radak #2: This is a metaphor. They will not exert or weary on the road. Normally, ascending and descending on the road wearies a traveler. Hashem will strengthen them, so they will not weary.
Why does it say "u'Mesilosai Yerumun"?
Rashi: Above, it says "Nashamu Mesilos" (33:8) - they were ruined, and no one fixed them. They will be fixed and elevated properly via smooth stones and earth.
Radak: It says [Harai and] Mesilosai, i.e. Mine. Hashem speaks; He will make new paths in the Midbaros where there are no paths - "v'Hayah Sham Maslul va'Derech..." (35:8).
Malbim: The path will be paved, so all will know that they are My paths.
