How to train for a duathalon relay?

Ok, so I've just been entered in the Treadman Duathalon on Sep. 24th in Pine Island, MN as the runner in a relay team, so I will be responsible for two 5k legs. I was wondering if anyone has any advice on how to effectively train for this event so I can post strong times. I haven't done any hard training in some time, yet I keep myself in "good" shape and am a former Marine so I"m not exactly starting on the couch, but this just happens to be my first formal race! .....Any suggestions?