The Dusky Trail is a pleasant back country ride, partly on singletrack and partly on four wheel drive track. Generally, but certainly not all the way, it's a relatively smooth ride on a grassy surface.
It makes a great out-and-back 21km ride to the picnic table at the high point and back. It's a steady climb up and a nice blat back down. There are two creek crossings, big enough to give you wet feet.
The trail also makes a good loop if you carry on towards Lake Pukaki and cross the Twizel River. After 23km you reach the Mount Cook highway (SH80), and from there you can loop back to the trail head on good roads for a 40km loop.
There is popular mountain bike race based on the circuit every 25 October. Google "Dusky Trail hard labour weekend" for more information.