Johnson is set to lock horns with his former team Manchester City in the Wembley showpiece and he knows Sunderland face a tough task, but it is one he is not afraid of.
"For me, City have the best squad in the league by far and it's going to be difficult, we all know that," said Johnson.
"But we have beaten them once already this season and we have had quite a good record in recent years against them.
"I remember being in the City side a few times coming here and struggling, so hopefully it will be like that again.
"We all know it's a massive, massive task for us, but it's a cup final and so many times over the years, it's not as simple as everyone expects it to go."
Johnson has been in great form in recent weeks, scoring seven goals in as many games in January and February, although he failed to earn a call-up to the England squad to face Denmark next week.
His improvement went hand-in-hand with Sunderland's climb from the foot of the Premier League table, but he insists there is no secret to the progress made both individually and collectively under Gus Poyet.
He said: "It's just been down to hard work for both. It hasn't been one thing with me personally.
"With the team, we have been working on all sorts of different things really - our shape, our attacking, defending off the ball.
"We have just been slowly putting it together piece by piece and it has started to show in our performances.
"It's just been down to working hard on the training ground, really, because we have the players. It was just getting that out of them."
Watch Manchester City v Sunderland on Sky Sports 1HD - coverage begins on Sunday from 12.30pm.