The Robins lost 1-0 to Hartlepool at Ashton Gate on Saturday, and Johnson feels his side need more stamina.
"I believe we can be fitter, and that is something we will be working hard on during the next fortnight," he said.
"We dominated the game against a team who didn't want to attack us - when opponents sit back, you have to be a touch more clever around their box.
"I want us to be able to steamroller sides for 90 minutes. I don't think we are currently capable of doing that."