Welcome and an Interesting proposition, I drew a straight line from my home town to the welsh coast. In my home county the straight line did not coincide with rights of way, then I managed to mould my route to that which was possible, that which suited the terrain and that which created a way, which fell in with the ideals I wanted the walk to reflect.
Looking at maps, so easy today with so much free mapping available on line. I suggest drawing a straight line on a road map east to the crossing of the River Severn, then look on more detailed maps where it will take you. As you start to walk the terrain starts to reveal itself, and the way starts to make sense.
It will never make sense if you try to jump from one national way to another.
I wish you pleasure in your enterprise, I enjoyed my walks following an idea.