23km Bike ride what sort of time?

If you don't already know how fast you can do 23 km, it may not work out to be your best event. The speed you can go depends on the nature of the course, it will be slower if there are hills and headwinds. A good time trialist should be able to average about 40 kph, but in a triathlon, you also have to think about the run, if you go too hard on the bike, you may have nothing left for the last section. If you know where you are going to compete, get out and ride the course a few times to get an idea of how to ride it, where to push and where to save energy. The time you take to do the course may be less important than how you ride it