If you Google Northumberland County council definitive map it will give you all the legal byways ect. I just mark them on my landranger map's when I go out to check there legal .
There's loads around your area. try going up to Slaley forest it's mostly easy tracks and there signposted . There's a good one going down to Blanchland
I have all the maps for Northern England your welcome to borrow them. It's worth checking the county council definitive map online first to check there's no TRO's etc .