Distances

[Meneltarmacil]

Actually, after seeing the movie a second time I noticed that by the time the army had reached the Black Gate, Frodo and Sam were already climbing the slopes of Mount Doom. So they probably took a few days to cross Gorgoroth in the time it took the army to get to the Black Gate. Plus it also helped that they took the direct route from Cirith Ungol rather than taking the long way around like they did in the book.