Is the wind consistant? does it blow the who;e day Is the 100km as the crow flys? if so real distance +40% dependant on wind angle How much chop is expected? Open water speed is generally below 30knots. 100km you will need check points/refueling stations.
It's easy enough to windsurf 100ks in a day, or even an afternoon - done it several times, as have plenty of people here.
One thing you might consider is that sailing that far down the coast could take you into a different weather zone where your chosen kit may be unsuitable. 60 k down the coast you can't just come in and change down/up. Perhaps a local marathon might be better? Just a thought.
There are a couple of threads in the NSW forum where this sort of sailing is being discussed. One valid point raised is back in the day, in the 80s, it was not unusual for windsurfers to try to go for longer distances and go on more epic journies. Its pretty rare nowdays. Perhaps its because back then they used longboards and sailed in lighter winds.
I could imagine windsurfing from the upper Central Coast down to the northern beaches of Sydney would be doable on the right day, a moderate seabreeze up to about 20 knots. Any stronger and it would get pretty difficult if you started out on bigger gear.