further to my last post, you would need to check out state of forests in case of extensive windblow. Google Earth would show clearfelled areas which would be best to use and you could check with Forestry .
Between Longtown and Kershope you may have to use minor roads on south of river Esk. I am not sure if you can use Kirkandrews Bridge which would allow some road avoidance by going north of river

From Metal Bridge there is a route south of the Esk on the flood bank to Longtown. You may need low water. Then there are lanes and paths towards Kershope and forests tracks and trails from there to the Pennine Way and St Cuthberts Way
Western section most difficult. Supplies will be a problem
Use the on line OS 1:25k with green space and cycle paths selected. You really need a computer screen rather than a phone but you may mange with the phone
Challenging, good luck!

I'm 10 days away from reaching Gretna on my round England wander. Any help with a route along the border would be appreciated, I can find little of help online and footpaths appear few, a fortnight of road walking is unappealing. Neil