Sociedad missed out on a top-four finish in La Liga last term having made the Champions League the season before, but Castro is in a positive mood ahead of the new campaign.
"The fundamental thing now is that it's pre-season and our physical work is going to form the base for the rest of the season," Castro told the club's official television channel.
"When the season starts, we're hoping to be fully recovered. We have important games at the end of August, with the qualifying tie in the Europa League and against an opponent who isn't going to gift us anything.
"We need to be in good physical shape and good form with the ball too, because whoever we face, they'll have the advantage of having already played competitive games in the previous round of the qualifiers. The first competitive game of the season is always the most difficult."
Sociedad suffered a 3-1 defeat to Ajax in their first pre-season match of the summer.