The usual way to ride the W2K is out-and-back from Kinloch. It's 24km or about 3 hours on the way out including the detour called the Headland Trail (not to be missed) and coming back it is 14km or about 2 hours taking the shortcut rather than going round the headland. That makes a total of 38km or about 5 hours. Take a picnic and make a day of it.
Alternatively, ride out from Kinloch and get a shuttle back from the eastern end of the trail at Whakaipo Bay.
Starting from the Kinloch car park you'll pick up the markers around the marina, along the waterfront and then up into the forest squeezing past some huge volcanic boulders. The trail is a gentle masterpiece, the scenery stunning, the forest is dappled shade, the vast expanse of the lake a marvel and the volcanoes way off to the south are a magical sight on a clear day. The views get even better as you go round the headland.
As you approach the end of the trail it descends through pasture to the beach. You can swim there but if you're heading back to Kinloch you might just turn around before the descent.