Depends on what kind of work he's doing. He could very well just be doing endurance exercises and working on stretching for muscle elasticity and flexibility, to further reduce the chances of muscle injuries.
He's very clearly not only looking at the rest of the season for Barca, but also preparing for the WC. He's taking the WC very seriously it seems. I just hope the other Argentina players do the same, because despite all his preparation, Messi's going to need all other players performing well to have a shot.