OP, it strikes me that you'd be better off posting on a disability and/or walkers forum rather than here, as you seem to have very specific needs.
Walking 60 miles down a railway line is not my idea of a good time, the pp didn't describe what can be seen and whether it's dull or beautiful. You yourself haven't specified that you want rolling hills and points of interest or just a long, straight road to walk along to keep fit.
My experience of trying to find a mix is that it's almost impossible to have the countryside on your doorstep unless you're prepared to sacrifice access to amenities.
Whilst it's true that Raby castle is wonderful (and has additional features coming very soon), we were there in May and between the castle and the furthest away parts where the deer are, there were only two benches, and I really, really struggled because M.E. was completely beachin' at me that day.
I agree with a PP that Barnard Castle is lovely and charming (and it's surrounded by sort of moorland? Idk how to describe it), it's also considered a bit cut off in terms of accessibility to larger towns. That said, I was very pleasantly surprised by the number of buses I saw on pretty rural roads overall in the county; we get sod all where I live!
Another pp is correct that 160k won't buy you a house in the best parts of Darlington, but, imo, it will buy you a house in an area that's probably just about good enough and not on the worst end of the rough areas.
I have also repeatedly heard good things about Stokesley, but the last I looked it's not all that budget friendly.
>Also agree with going much further up the coast to Northumberland, stunning!
I would give this further consideration, I continue to hear nothing but good things from outdoor lovers re: Northumberland.
So, yeah, for specific needs, perhaps a more specific forum would be better placed to help you. I hope you find somewhere wonderful :). I'd love to hear where you chose to settle eventually.