I've actually studied the matter a fair bit, and have found that important fantasists had unusually strong emotional, intellectual, and imaginative connections with particular places.
William Morris was particularly attached to the relatively unspoiled reaches of the Thames, if I'm not mistaken. He also got to know Iceland at walking-pace and pony-pace when it was quite its own place.
Rider Haggard wrote with much love and knowledge of the area where he farmed.
Beatrix Potter's feeling for the Lake District translated into books and also into the rescue of landscapes from "development." Denis Watkins-Pitchford (BB), who wrote The Little Grey Men, etc., celebrated the outdoors and a free-range childhood.
Lord Dunsany seems to have been deeply involved with Irish locales where he hunted -- walking, of course. I don't remember much about T. H. White's boyhood, but certainly as an adult he was an outdoorsman.
Tolkien and Lewis had their walking tours. Tolkien's profound connection with certain parts of England is well-known and integral to his conception of Bombadil or even of Farmer Giles. Lewis's published letters (and the portions of his brother's diary that have been published) reflect their love of the countryside, walking, slow trains, villages, etc.
Lovecraft's devotion to colonial New England is well-known too, btw. for what it may be worth, his penpal Robert E. Howard seems to have been immersed in the tales and landscapes of his own region, and Clark Ashton Smith lived in a backwoods cabin much of his life; however, I don't reate those guys with the other authors mentioned here.
Richard Adams's Watership Down and The Plague Dogs, etc. could not have been written without his devotion to an intimately known walker's world.
Garner is also a case in point. I'd be surprised if Ursula le Guin had purely urban youthful years. Lars Walker grew up on a farm.
And so on.
I suppose someone like Fritz Leiber was always an urban fellow, but then his semi-heroic fantasy can seem to me more a half-spoof of fantasy. The Leiber who gets through to me more is the fellow who wrote the very urban You're All Alone (The Sinful Ones), which is more in the horror vein than "fantasy" as we usually call it.
These remarks are something of a footnote to my earlier remark expressing doubt that we shall have much more Tolkienian fantasy -- though we may have many trilogies and so on with magic talismans, elves, dragons etc. These will be more like Peter Jackson's idea of Tolkien than Tolkien.