What is the meaning of "va'Yinag'u"?
Rashi: It is like the Targum (v'Isberu). They made it look like they are stricken in front of them.
Radak: They made it look like they are stricken and fleeing, and they were stricken. Had they merely feigned to be stricken, it would have said va'Yisnag'u (the verb form Hispa'el). Malbim
What is the Midbar?
Radak: It is where animals graze, whether it is close to or far from the city.
Malbim: This is Midbar Beis Aven (18:12) between Yericho and Beis Kel, opposite Ai, to the north. Yisrael retreated backwards, to the north.