I went on a hike from Liddington to Overton Hill over a weekend a few years back, but from Averbury you can walk all the way to Ivinghoe Beacon near Wipsnade in a week or so. Overton Hill is recognised as the official 'end' but it can be traced much further south into Dorset & Somerset. I didn't know it went as far as Cornwall but it wouldn't surprise me.