Definitely best if you can hook up with some locals – try the Timber or Thetford MTB racing sites for starters – both friendly groups with regular rides. There is lots and lots of great unmarked singletrack – plenty of it is easy to find if you just follow your nose, but there are lots of dead ends too so stringing it together into a decent loop is a bit trickier without some local knowledge.
There’s a trail conditions section on the Timber site – mostly the trails drain fairly well as the soil is sandy but some sections on marked routes can get pretty churned up so are best avoided if wet.