In reply to J Brown:
As an Edinburgh man I'd also recommend anything in the Pentlands - good running, easy navigation etc... but it hardly breaks the journey as its pretty much at destination.
Think about Moffat perhaps... that's about half way. If you take the road from Moffat towards Edinburgh, a few miles North once towards the top of the Devils Beeftub you can park up and head off towards the east. I'm pretty sure you can put a nice circuit together there.
Easier and more direct drive than heading to the Grey Mares tail and through to the A7 as well.