Check out LandRover World classified for lists of purpose-built off-road tracks – usually easier to navigate than green lanes.
But whatever you do, be sure to have the kit you need – off-road tyres, elasticated bungee rope, shovel, winch etc or you will end up looking a knob one way or another.
(remembers watching a guy bin his new Land Cruiser sideways sliding into a tree at Tong near Bradford as we sailed by in our 40 yr old LWB Landy that actually had suitable tyres on.
Ohh, and learn how to drive correctly off-road if you have never done it – there are particular tricks to doing it safely.