In reply to Mark Bull:
> I have not done it, but there is a roughly 7 miles circuit from Byrness following the Pennine Way to Raven's Knowe and Ogre Hill, then back through the woods past Spithope Bothy.
There's a longer version of that route here
https://www.nationaltrail.co.uk/en_GB/short-routes/walk-way-day-walk-06-upp...
I have not done this (either version) but I think if I had time I would try and get as far as the Roman Camp at Chew Green.
+1 to going over the Eildons.
If you've never stopped at Carter Bar, the view is worth stopping for, IMHO.