I'm pretty sure the Spurs coaching staff prescribed a special stamina program to Mason.
he ran out of gas in the middle of last season (IMO it was this fact that caused Pop to try to play him at backup PG, it was a try to reduce the workload).
he never played as many minutes as last season and he has never been an outstanding athlete to begin with. the mix of reduced weight, better shape and minutes in the 20-25 MPG area should (hopefully) help him to survive the regular season and be effective in the PO.
the question will be, who can defend the strong SGs. (even a heavier Mason wasn't able to do this well last year). this is one of the remaining holes of this roster. RJ will be needed to guard the athletic SFs. maybe Hairston helps in this department, but I don't think he is ready yet.