I live in Northamptonshire, near Corby, and a lot of the walks are based in the county.
I used to think it was just a place you dashed through on the way to somewhere more interesting, but there are reservoirs, rivers and canals with easy access - for walking, fishing and boating. There are still quiet country roads where you can cycle. Lots of the villages have gingerbread-coloured ironstone houses as pretty as any you see in the Cotswolds. There are stately homes set in delightful rolling scenery. The county is soaked in history and not a little blood shed in long ago battles.
It's not high ground - the highest point is Arbury Hill at 738 feet, but the short climb is worth it for the view.
The Nene Way is a great walk following the course of the river from near its source in Badby, through Northampton, Thrapston, Oundle and Peterborough. It continues through Cambridgeshire and eventually reaches the Wash at Sutton Bridge. 70 miles of the walk lie in the county of Northamptonshire. Of course it follows the low ground most of the way, but visits some of the villages above the valley too.
For a contrast the 88 miles of the Jurassic Way from Banbury to Stamford follow the ridge of Jurassic limestone through Northamptonshire.
I shall link to some of the walks in Northamptonshire, or you can search the blog using the label Northants.