The rooty trail next to the stream is great, but there are plenty of others. I'm partial to the one that starts off on the section of boardwalk near the farm and eventually deposits you on the edge (northern edge?) of the woods near the road (apologies for the vague description, I don't know the names of any of the landmarks!) From there you can take the Roman road (Ermine Street?) over to Hertford Heath, where there are more trails.
Everything I've ridden has been north of White Stubbs Lane (when I used to drive up I'd park in the car park just off the sharp bend on Pembride Lane:
https://www.google.co.uk/maps/@51.74510 ... zA4hSA!2e0) Looks like there's quite a substantial area to the south of that too.