It depends what you consider a reasonable distance. There's a load in Northumberland, but for the most part they are relatively short and not very taxing (I've driven lots of Northumberland's BOATS in a transit for work)
There's a couple of longer ones around Hexham and Haydon Bridge I haven't tried yet that look like they might be slightly more challenging.
Another good point about laning in Northumberland is that the council keeps its definitive map online so you can check the status of lanes before you drive them.