United meet Carlisle in the play-off semi-final, with the first leg at Elland Road on Monday, May 12 (7.45pm), with the return at Brunton Park on Thursday, May 15 (7.45pm).
Johnson, who scored from a wonderful volley in Saturday's 2-1 victory over Gillingham, said: "Everyone is focused. Everyone has done so well to come back from the 15 points and we just don't want to throw it away. I think we can go all the way, but we have to prove that when we play."