Did it the other way in March in 3 days.
Did it to have a look at the route of the LEL Audax that I'll be doing in July.
Day 1: 320km Putney to Thorne (
Travelodge at M18/M180 services)
Day 2: 220km Thorne to Alston (B&

Day 3: 210km Alston to North Berwick (Travelodge in Edinburgh)
GPS ran out of spare coming in to Dalkeith.
Route looked like this:-
http://www.bikely.com/maps/bike-path/266092
If I did it again I would:
1. Stay on the A7 from Langholm to Longtown. The Canonbie diversion was to see what the LEL route was like. It's hilly.
2. Avoid the A66 to Scotch Corner, this was sheer laziness as I couldn't be bothered to retrace to avoid the busy road after going to the services.
3. Not miss the turn between West Stockwith and Haxey. See annoying long cut.
The B709 is a lovely road. Hilly but the climbs aren't steep.