I know this is a late reply. i'm new here.
Thought i would post incase the above user is still interested, or for any new members.
I'v only ever done a couple of short walks in Northamptonshire. (I'm in Leicestershire)
Harlestone Heath and Dallington Heath was very nice, with a network of paths through the woods. A very nice tea room as i recall.
Also the woods at Maidwell were very nice. i remember wishing i could spend longer here, we only had a few hours, and didn't have an map, so not sure how extensive the rights of way in the area are. Well worth a visit.