This article about purposeless walking on the BBC website struck a chord with me.
I'd recently been thinking about how mentally refreshing walking around some of the remote headlands on Shetland was compared with, for example, rural walks around Oxfordshire, and was trying to figure why there was a difference.
I think the difference really comes down to how easy is to give your mind the opportunity to wander at the same time as you're giving your body the chance to wander.
When the space is open, as it often is on Shetland, it's possible to allow your mind to do it's own thing. You might need to make sure you don't wander off a cliff edge or fall into a bog but other than that you don't need to spend too much mental energy figuring out the route you should be following.
In a busy farming area, like Oxfordshire, you are going to be constrained by footpaths and field boundaries, and the signs intended to keep you in the right direction are unfortunately often very badly maintained or increasingly just missing. This means that you need to spend a lot of mental energy just doing the basics like figuring out where to go next and trying to ensure that you don't stumble into an irate, and occasionally, shot-gun-toting farmer.
I think this observation also leads me, when I'm in southern England at least, to seek out river-side or canal-side walks when I can. At least with a feature like a river or canal you can set aside some of the navigational overhead, and just let your mind meander.
The downside is that it can be a real challenge figuring out a circular walk based on a river or canal, but that's another problem altogether.