I cannot read McCain's mind, but I can give you my opinions and impressions.
First, regarding "get as far away from Bush" - I think many Republicans were disappointed with Bush precisely because he departed from his conservative agenda and increased federal spending.
As to McCain, although he did win the primary rather easiliy, that was for a myriad of reasons, but it certainly was NOT because most Republicans preferred McCain. In fact, specifically for the reason you mention (he was seen as leftist or centralist), there were many, many Republicans who had decided to sit out the elections and not vote. One very vocal voice was Focus on the Family's Dr. Dobson, who said he simply could not bring himself to vote for McCain. Another frequently heard opinion was, "I am going to go into the voting booth, hold my nose, and vote for McCain."
Then, McCain chose Sarah Palin as a running mate. Suddenly, there was a renewed energy and enthusiasm in the Republican Party. Dr. Dobson reversed his view, and now says he will vote for McCain. In the Sarah Palin thread on these message boards, other conservatives have expressed the same thought - they were not happy with McCain, but they will enthusiastically vote for a McCain/Palin ticket.
I think the reaction of the Republicans to Palin got the message across to McCain. Based on the overwhelming response of the party to a true conservative, McCain has realized that this is what his party wants. They want a conservative, not a moderate, not a centralist, not someone who is liberal-leaning.
In my opinion, another incentive for McCain to lean right is the ever-growing wealth of evidence showing that Obama is extreme left. To the degree that Obama is on the very far left (boardering on or actually crossing the line between "liberal" and "socialist" - at least as defined by Americans, if not by Europeans), McCain can lean a great deal more to the right than one might ordinarily expect and still be closer to the center than Obama is. Thus, he can corner the Republican market, so to speak, while still representing a viable option to right-leaning/centralist Democrats who simply cannot agree with Obama's vision for America.
Obviously, from what I've written, you can see I disagree with your opinion that Obama comes across as centralist. Obama comes across as someone who will say whatever he thinks his audience wants to hear. His true views, however, are very, very left-wing, and it is becoming harder and harder for him to hide them.
