I'm in Ockendon as well and we used to do the Thurrock loop (meet at Bannatynes Gym and then do a 25 mile loop up through Ockendon/Upminster/Rainham/Thames path). It's about 75% off road.
In the area you have as you have noted Thorndon - there are normally rides there Saturday/Sunday mornings that leave from the Kart Track carpark.
I don't know of a formal ride at Langdon Hills but they come up from time to time.
There are loads and loads of trails locally - best bet is to get on Strava or Endomondo and look them up - they are easy to follow.
If you keep an eye out in the Ride Calendar section of the forum you see stuff come up