Here's a good read on the training the actors of '300' were doing.  They got ripped quickly, and no, it wasn't on steroids or with CGI.  Basically, they worked hard and constantly shifted their activities while controlling their eating.  Basically, they did some of the same things the tadpoles do in BUD/S.  I went to mini-BUD/S during one of my midshipman summer cruises.  I looked a lot more muscular when I left after just 4 weeks, so it seems entirely reasonable that these guys accomplished what they did in 8.