Yes the B6344 is closed but what can do to get to Rothbury is continue on the A697 through Longframlington (watch your speed as there are speed cameras there and also at Longhorsley which you would also have travelled through). A few miles after Longframlington turn left onto the B6341 - think signposted for Rothbury and Cragside (which is a fantastic NT property).
Follow the B6341 through Rothbury and continue through Thropton (speed cameras as well I think).
Continue on B6341 and about 3/4 miles further on (approx) there is a right turn signposted Sharperton, Harbottle and Alwinton. At Alwinton over the bridge and go on until you need to take go left, go past the pub and car park and straight on to the single track road and watch out for the sheep.... ..Barrowburn approx 6 miles down this road,
ps Cheviot has been mentioned - if visibility is good also consider trying Hedgehope as generally agreed better views. Nearer Barrowburn Wyndy Gyle good views as well IMO.
Just a thought - don't know if this is any good for approx a 9 miler (some Hills though) but can go from Barrowburn, up The Street to join the Pennine Way. Up to Wyndy Gyle. Back over into England and continue on Pennine Way to Clennell Street. Down Clennell Street but leave it to go up Middle Hill (otherwise you'll end up at Alwinton) then back down to Barrowburn.
Not much help I'm afraid but we were walking past it a couple of years ago and a mate wanted a look inside - door was unlocked and unoccupied - from what I can remember then it was perfectly ok and my mate said he would have been happy to stay there.
Totally agree about the location - I don't get out that much at the moment due to other commitments but that area is one of my favourite in Northumberland - will park half a mile further up the road at the stream at the bottom of 'The Street' and up to 'Wyndy Gyle' is a favourite walk.
Simon - you asked if energy/protein bars etc are a myth or not. There was a Panorama programme last year looking at the evidence about various sport products and whether they work or not - It is still available on the iplayer called 'The Truth About Sports Products' interesting programme.
Graeme Obree mentioned by Peter earlier on is on it a couple of times discussing his diet - including the jam sandwiches.
I'd experiment with various things and see what works for you and stick to it whether energy/protein bar or not. For me a few bananas do the job.