"The paths are long."
Translation:Die Wege sind lang.
Is this sentence accusative?
Sentences aren't accusative, or dative, or any particular case.
Parts of sentence are in a case, to show their role in the sentence (e.g. subject, indirect object, direct object, expression of time).
The verb in this sentence is a form of sein "to be". This is a linking verb (copula) that links a subject to a predicate that says something about the subject. Both the subject and the predicate are in the nominative case. werden "to become" is another verb that acts similarly.
There is no direct object in this sentence and nothing in the accusative case.
Most verbs that "act on" an object have that object in the accusative case.
But some verbs take an object in the dative case, e.g. helfen (to help) or folgen (to follow). Those have to be learned as exceptions.