I don't know where you guys got the idea that Superman was buff. Take a look at the pictures of all the actors who have played superman. None of them in are in exceptional shape.
If you saw what Tom Hanks went through to lose weight and get in shape for "Cast Away", I have no doubts Tom can reach any level of fitness to play Superman.
http://www.fiftiesweb.com/tv/superman.htm
-Tony