Digging up this thread - how rough is too rough? I've plotted a fantastic route which is about 100 km from Northallerton, or a bit overdistance from my likely start/finish point. Includes Thornton-le-Beans and other places with names nearly as good.
However - the track up Monket House Bank between Farndale and Bransdale is much rougher than I remember, as it's popular with 4x4s and scrambling bikes. It's safe to walk, but you'd have to be a very strong rider or on a MTB to ride it. I was overgeared and underskilled, but once up on top, it was glorious, despite the rain. The photos don't show the worst of the climb - it was pissing down, so I was reluctant to dig out my phone.
There's also a really tricky descent to the hidden valley of Bransdale:
However, after that it's pretty much ridable on anything, though narrow tyres would be a bit squirmy.
I'm tempted to put it on the calendar, and just give people loads of advice about alternatives to dodge the nearly-unridable bit between Farndale and Bransdale (which is pretty easy along quiet lanes, but longer and probably hillier). I reckon it'll live up to the Brevet Unpopulaire tag