Translation:It is not far.
「だ」like other particles 「ね」&「よ」do not make a big difference, but they add a tone to the sentence. In the case of 「だ」it makes it sound more like a declarative statement. Though you cannot use it after an 「-い」adjective.
About 「-ではない」 as far as I can tell it feels quite formal, but it can get more formal with 「-ではありません」