In addition to the great tips above...
If you have not practiced a transition,here is what to expect the first half to full mile of the second run to be very disorienting and awkward. Changing from circles to a running stride will make your legs feel very fatigued (like bricks) and want to flat quit. Pedaling in a lighter gear for the last half mile and the transition in run can help, but it you will feel it.
If you decide to run a few more du's or tri's make a brick workout a weekly or semiweekly workout. Simply lace up and take a run after an intermediate cycle ride. A good distance for the brick run is a 2-4 mile run. Your goal is to develop the mental toolset to run through the awkward brick feeling in the legs.
One last thought on a first DU strategy....
If you think you will have a high set of nerves and have clip pedals, consider installing flats on your bike for the first du. It's not hard but having been through a few bike outs I have seen several wrecks from fellow cyclists having a difficult time clipping in at the start of the bike. There are lots of advanced ways to get out and go on the bike. Keep it simple... run past the mount line head to one side of the course and mount like you would for a regular ride. If you do go with flats, make sure to tuck you lace loops under the laces of your shoes pre race - you do not want to have them tangle in the crank.
That leads to another thought... Make sure that you are in a go gear when you place you bike on the rack. Look at bike out and if on a hill, you may need an easier gear than you left the bike in last time used.
Most importantly - have fun. It's a great sport