The most common advice is swim a bit, run a bit, bike a lot.
But as I said, you do need to be able to cover the distance comfortably.
Why not have a look at last year's result if on the web. What sort of times were people doing for the different disciplines? What sort of times can you do? Where can you make most improvement.
Sounds like you did a brick session the other day (bike then run straight away) - it is important you do several of these as it can be really tough to get the legs moving after the bike if you haven't practised it.
You should also practice transitions - you can waste a lot of time there otherwise! It is amazing how your hands don't do what you want when you are just out of the water.