Game setting
In deciding on a setting, I’m going to try and follow the old adage “write about what you know”. As a guiding principle this should help me make the setting believable, and to generate ideas for encounters and the world. Now for the place and time…
Place
I enjoy both kinds of games: dungeon crawls and overland adventures. 
Having no life experience of dungeons, I’ll go with an overland adventure.
An overland adventure in a place that I know? There is no wilderness in modern Britain. Its a tiny island that’s been populated since the last ice age. The closest I can get to a wild country is the Peak District national park. Even then its peppered with villages that have been there since at least medieval times.
The Peak District forms a breathing space between the cities of Manchester (my side) and Sheffield, and now has millions of visitors every year. It only covers 555 square miles, so is not a vast expanse, but should be big enough for a bit of adventure. In the modern world it could be circumnavigated quickly in a car. But even a couple of hundred years ago it was wild enough to require a local guide.
To quote Daniel Defoe: “This, perhaps, is the most desolate, wild, and abandoned country in all England.”
The Peak District also has hundreds of remains of stone circles, cairns, barrows, henges, standing stones, and hill forts to act as evocative locations.
It is also the 75th anniversary of the national park this year. Happy birthday.

Time
The further back in time we go, the more untamed the country becomes. But I also want the Bronze and Iron age barrows to be haunted relics of a bygone age - much as they are today. I also want to stay far back enough in history to avoid gunpowder weapons.
I think sub-Roman Britain gives a good reason for the area to be wild and under-populated. 410AD is often mentioned to be the date that Roman legions withdrew from Britain, leading to the rise of small tribal kingdoms. This is a reason to define the boundary of the realm on the map.
Historians might dislike the term Dark Ages, but it is an evocative term for game lore.