The long bridleway from Keston to Sheepbarn Lane is a good one. If you go straight across the road there's another bridleway through the woods, then across the fields to link you up with Skid Hill Lane. You can then follow NCR21 to Woldingham which has a good mix of off road and road sections.
Also check out local bridleway network (Keston through to Biggin Hill, Downe etc.) and woods around Sidcup, e.g. Joydens.