The Black Cats survived relegation by just one place last season, but the former Manchester City winger believes that they can surprise a few people this season.
"We had an idea towards the end of last season that pre-season was going to be hard under the new manager," Johnson told reporters.
"We knew it was going to be all about getting our fitness levels up because that's going to be a feature of the side he wants us to be.
"I think we're looking fit and strong now. There's a quiet confidence growing in the squad and a much better idea of what is expected of us."
Sunderland begin their new Premier League campaign at home to Fulham.