The last question in the first paragraph is correct. Tolkien had no idea the Necromancer was Sauron when he wrote The Hobbit. A lot of people wrote him after it was published asking for information on Gandalf. What were these other errands he had to tend to? Who was the Necromancer?
At the request of all of these readers and the publisher, Tolkien started to write The Lord of the Rings. Much of this can be seen in The Letters of J.R.R. Tolkien. The letters in chronological order deal with almost everything you could imagine - correspondence with publishers, critics, and other authors/friends (Auden, Lewis) as well as fans and family members. In the first 100 or so letters, Tolkien is dealing with the success of The Hobbit and the beginnings of The Lord of the Rings. He shares a lot of what has happened as he is writing with his son Christopher.
He never ever intended it to be such a monster of a tale.
I must be going now, but I'll get back on tonight and add quotes from various letters showing the work's progress as a whole and Tolkien's thoughts as he saw what it was becoming.
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...take counsel with thyself, and remember who and what thou art.
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