Translation:Marcus makes a long journey to the city.
Iter is a neuter noun, so it is in the accusative case along with longum. Urbem is in the accusative because it indicates motion towards rather than a location within the city. I am not an expert though, so I may not be right.
Exactly. And urbem is accusative because it is the object of ad, which takes the accusative.
Neuter nouns have the same form in the accusative and neuter. How do you tell them apart in a sentence? It usually comes down to a question of what makes sense.