Thanks to everyone for support, advice and help with the trip, which ended successfully on Wednesday on a glorious day in Unst. It all just proves that a fat 56-year-old fond of a pint and a curry can do 450 miles without serious training...
Couple more things: Sustrans got back to me, politely pointing out that their routes have to be suitable for 'anyone between 8 and 80'. Alas, I fear parts of NCR1 and the canal towpaths simply aren't.
And the LHT may have been slightly too big, right enough...but last minute substitution of my 20-year-old B17 for a newer one solved all problems.
Finally (that's three things, I know): the infamous A90 Stonehaven problem. There IS a cycle path by the side of the dual carriageway, on the east side, between Stonehaven and Portlethen, but it doesn't start until the golf course turnoff, and it's informal for the first mile, ie, narrow dirt. There is NO path between the golf course and the A92 turn off.
More, and pix, at http://mull2muckle.blogspot.com