More info needed. What is your background? What are you good at? That is a hell of a short duathlon. For a fast guy that is a 50min race.
The one workout you MUST do is a "brick" workout...a hard bike followed but a fast run. The hardest part about a duathlon is the transition off the bike and into the second run. If you can learn how to do this well through practicing it a few times it will really help.
It really depends on if you are a biker or a runner. If you are a runner, you want to hit the first run hard, basically bike as hard as you can to avoid getting behind, then rely on your run ability to get you through a tired second run. If you are a biker, you want to run a bit more controlled on the first run, hammer the bike as hard as you can, then do as best you can on the final run.