The Tigers will return to the Premier League following a three-year absence after they won automatic promotion from the Championship last season.
Bruce told the club's official website: "All of the players have come back to work in really good shape and they've all been doing some work during the break. That is good to see because they will need to be at their best physically and our aim now is to make sure they get there – this is a vitally important time.
"It's been a sweltering start to pre-season with the weather and you can imagine how difficult it is for the players in the heat. But it's vitally important for every player that they get a pre-season under their belt, it's like money in the bank for them."
Bruce led the Tigers back into top flight in his first season in charge at the KC Stadium.