Sunderland's players have had double training sessions since early July and Johnson says boss Paolo Di Canio's tough regime will reap rewards.
"We had an idea towards the end of last season that pre-season was going to be hard under the new manager," said the winger.
"We knew that it was going to be all about getting our fitness levels up because that's going to be a feature of the side that he wants us to be.
"I think we're looking fit and strong right now. There's a quiet confidence growing in the squad and a much better idea of what is expected of us."
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