"Our journey is too long."
Translation:Nasza podróż jest za długa.
You can always use zbyt, and you can use za only before many, but not all adjectives.
You cannot use za-adverb after za-preposition (za za would look and sound silly).
You cannot use za before participles.
You cannot use za before negated adjectives.
I'm confused as to why the other translations, "zbyt dluga" and "wyprawa" are not acceptable.
Oh, there were another version in among the options. I dont remember it and I wrote it bad xD.
You probably meant "długiej", but that's a wrong case (Genitive singular, here you just simply need Nominative).