Go for it ! It sounds like you have come along way with your running, Isn't that a great feeling?! For your first duathlon I would suggest finding one that is a "fun" one, with lots of newbies. You will be amazed at how nice people are at these events if you are new. One thing that I tried to practice was riding and running right after, even if only for 5 min, to get your legs used to the shock and to get an idea of the transition time and how I could go faster. Remember that you are there to see what you can do, have fun and next time you can worry about kicking butt. Good luck