In Maniac Magee by Jerry Spinelli, where are Piper and Russell running away to?

In Chapter 34, we learn that Piper and Russell are running away to Mexico.
As the chapter begins, Maniac discovers both boys in the cabin next door. When he first sees them, Screecher and Missing Tooth (this is Maniac's first impression of them) are engaged in a brawl of sorts. Both appear to be enjoying the rambunctious fight. When both boys notice Maniac standing in the doorway, they address him and ask whether he's running away as well.
Maniac replies in the negative, and both boys readily show him their stash of stolen goodies. Eventually, Screecher and Missing Tooth (Piper and Russell respectively) become suspicious that Maniac might be a cop. They pat him down for a gun but discover nothing. For his part, Maniac pipes up that he's just a pizza delivery boy. He thinks that "someone somewhere must be worried sick" about Piper and Russell. So, he tries to bribe the boys to return to Two Mills with him by telling them that they've won a free pizza. He also promises Piper and Russell that, if they consent to return to Two Mills with him, he will also clue them in on a shortcut to Mexico.
After the trio finish their pizza meal, they try to make their way home. However, they are soon accosted by Big John McNab, the leader of the Cobras and the boys' big brother.