Most of the trails are firetrails for National Park access but are open to Mountain Bikers and walkers. There are some walking only tracks so take care. There are also lots of unmarked single trails around for a bit of additional and more technical fun.
Suggested route would be to start at the trail head off Britannia Rd opposite the hockey field, head down the trails east until you get to the Glouster Ave access point in West Pymble and then head back the way you came, looking for alternative trails along the way. There are good maps posted at various points on the trail to help you keep your bearing.
This is a good track for beginners as its mostly nice and wide but also some good hidden technical areas for the more adventurous. The only thing is that is always a climb out of the valley at the end of the ride.