Use of "Longus" in Lingua Latina Cap. XII

Salvete sodales.

I have a quick question about the use of the adj. “longus” in Chapter 12 of Lingua Latina Per Se Illustrata. I am reading through the text mostly as review, but I am unsure of what to think of the word here:

“Marcus: ‘Quam altum est vallum castrorum.’ Iulius: ‘Prope decem pedes altum est, et duo milia passuum longum.’” [M. ‘How tall is the wall of the camp’. I.: ‘It is near ten feet tall, and 2,000 passus long.’]

Is “longus” used here as an adj. in agreement with “vallum” or is it being used almost adverbially in the phrase “2,000 paces long”? I suppose I got the gist of it and didn’t question the grammar of the phrase until Pensum A of this chapter which uses a feminine noun with the “mille” construction:

“Via Latina CL…milia pass- long- est.” [The Latin Way is 150,000 paces (passuum) long.]

This is where my question about “longus” came up. Should I write that the Via Latina is 150,000 paces “longa est” or “longum est”? I am not sure if “longum” is some sort of adverbial use of the neuter instead of a simple adjective ending.

Multas gratia vobis ago!

In the first sentence, altum and longum are adjectives adjective modifying vallum. Decem pedes and duo milia [passuum] are accusatives expressing what Allen & Greenough sec. 425 calls “extent of space.”

http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0001%3Asmythp%3D425

So the second sentence should be Via Latina CL milia passuum . . . longa est.

Salve Qimmik,

Thank you for your thoughtful reply and use of Allen and Greenough (especially the acc. of space!). I thought I was overthinking “longus”, but I wasn’t sure since the construction only occurred with the neuter in the text.

All the best,
Zach