The very last section from Sea House (not Seahouses) to Berwick is a bridleway across a field.
It will be quite wet and muddy at this time of year , unless Sustrans have upgraded it recently.
You can tell before you set off across the field what it looks like, as it is the first bit that is worst IIRC.
There is an option to go up the hill and through Scremerston to join the old A1 and then its downhill all the way to B-O-T.
Most of the other off-road bits have gravel or crushed stone surfaces. Mrs. M was on 25 mm tyres with rear panniers when we did it a couple of years ago.