To be more detailed, I'd like to know what would compel a Roman citizen to join, an average military career, and what would their life be like afterwards. But I'd be pretty content with just a military career. Thanks.
Would you be interested in a response detailing military life in the 4th century A.D.? Late Antiquity's a bit removed from the time period you're thinking of, but it is technically post-Marian! :)