I did my first on a fixie. If you can do the distance, you'll be fine with that bit - fixie legs are powerful legs. Get comfortable with a little over the distance in each leg by then if you can.

Some bike/run sessions are worth doing - they get you used to the weird feeling of running right after riding. And if the swim is open-water and/or wetsuit, do a couple of sessions in those conditions too, because they're subtly different to pool swimming.

And have fun