If by Fit you mean " having a high condition for the next game", then its all about not training too much( perhaps rest players after a game), rotating the squad and use a tactical approach that the team can handle( not too much closing down, running with ball, runs from deep). Look at the kilometres they cover for a game and try to see a pattern between it and the condition the players are in at let's say the 75th minute. The biggest problem here is in most cases " running with ball" set to often for a player that sees a lot of the ball.