There’s a great railway trail called Marriot’s Way. If you find Halfords, there’s a path off the roundabout nearby (it goes along the side of Halfords, next to the river). It’s tarmac footpath for the first mile or two, then dirt track for about 15 miles or so out into the country. It’s Route 4 on the National cycle Network, very good fun and ideal for SS CX, but not challenging in the slightest (apart from a few funky ramps up and down the old railway embankments).
Theford’s good fun but even if you have a car it’ll be an hour each way to get there. If you’re on a train don’t even bother.
There are plenty of fun green lanes and paths and allsorts if you’re happy to head out on the road and get lost a little bit. I suggest aiming for Reepham or thereabouts (actually, Marriot’s Way goes up that way, maybe head out on that and then back south by road and random trail).