The county has a lot of lanes, but quite a few are currently closed especially around Little Downham/Aldreth/Coveney, opening up hopefully end of the month.
Get a few Landranger or Explorer maps. Memory Map is better still.
Or join the local (reasonably active) GLASS group and you can get access to the WayFinder database.
Beds, Herts and Cambridge Land Rover Club obviously cover the area and go laning much further afield.
The lanes are on the councils online GIS but it's just crap and their TRO info is just as bad.
I haven't tried any local Pay n Play site as yet, but from what I've heard Tixover (@Peterborough'ish good), Wild Tracks (@Red Lodge poor), Bures (@Bures closed to PnP?), Devils Pit (@Barton-Le-Clay extreme?) and 3 something/holes? (@Wisbech no idea)
Drop me a PM with your mail addy.