Wouldn't the core paths, which are being suggested by the link provided by the OP be those that are serviced by local authorities. Similar to the A list RoWs in England.
I do not think it is the paths DA suggests as difficult to get mapped.
What would be interesting would be the current distribution and and density of a core path network. My previous post is, I feel, in line with Agent Mancuso and Happyhiker. To have paths which are maintained and furnished, out of local funds, clearly visible on the OS Explorer Maps and Landranger is probably the nub of this issue.
Are there enough paths;
The England and Wales network may work out at about 2km/per grid square and in much terrain that this figure is served by footpaths at this ratio it is insufficient. But nationally 2 miles per square mile is really stretching local authorities budgets to furnish and service.
The England and Wales effectively have a 'core path network' we do not have the right to make that network more effective, Scotland has the right that is denied to Welsh and English walkers but do not have a 'core path network' to make the first steps towards creating an effective and general network of leisure routes through all their varying terrain.