http://xymphora.blogspot.com/Deep throat revealed
Joseph Cannon has two excellent posts on the 'revelation' that Deep Throat is Mark Felt. Felt may honestly feel that he was Deep Throat, but could not have known everything that Deep Throat knew. He is an excellent patsy to take on the role, as he has Alzheimer's and can't be asked any embarrassing questions.
I imagine the long wait for the identity of Deep Throat was a wait dependent on finding someone with the proper characteristics who could assume the role without having to provide details (i. e., either dead or senile). I doubt very much there ever was a Deep Throat. He was a fictional method of funneling the information that Woodward was getting from whomever he was really working for - likely CIA or Joint Chiefs of Staff - into the stories intended to unseat Nixon, or at least weaken him. Woodward needed a mysterious source to explain how he was getting material that should have been impossible to obtain, and a source who could never be questioned. Deep Throat was a brilliant plan (the name itself, in the context of the times, was brilliant marketing, as everyone got a cheap thrill by being allowed to say it).
We know the Joint Chiefs of Staff were spying on Nixon, and that Nixon caught them and let them get away with it. The American Powers That Be were apparently terrified that Nixon, who up to the end of the 1960's was a reliable, crooked, mob-connected political hack, was intelligent enough to realize that his place in the history books would be determined by the substantial good he did. China whet his appetite, and there was a very real danger that the old fool would succeed in approaching the Soviets and ending the Cold War fifteen years early (and billions and billions of dollars in weapons sales early), all in a bid to take his place in history. He had to be stopped, so the barely-literate Woodward mysteriously appeared at the Washington Post, was hooked up with a real, if spectacularly unsuccessful, journalist in Bernstein, and suddenly received all kinds of unexpected help from the Very Establishment Ben Bradlee. A newspaper that you would never expect to even consider challenging the status quo, and very connected to the CIA, was suddenly lauded as the king of investigative journalism, and the savior of the Republic. All nonsense, of course. Nixon's big character flaws, instinctive dishonesty and paranoia, were manipulated to slide him into a completely unnecessary cover-up of a completely unnecessary burglary conducted by a bunch of CIA agents (a burglary Nixon wasn't even aware of until after the fact, with the burglars conveniently getting caught through extreme bungling and conveniently having documented ties to Nixon's crooked political financing system), and Nixon was safely pushed out of office before he could do any real harm. Rather than being a victory for journalism, Watergate was the start of the systematic corruption of the disgusting American media, which continues to do its job in hiding the fact that the United States has been a military dictatorship since November 22, 1963.