I don’t think McCain can last 2 terms as this is a high stress job. If he becomes president, he will be the oldest at 72.
I was surprised to find out that McCain himself has said that he might not seek reelection:
If I said I was running for eight years, I’m not sure that would be a vote-getter
Note that McCain didn’t say that he doesn’t want a second term but rather that he thinks that citizens won’t vote for him now if he said that he is willing to serve 2 terms.
If McCain is elected and does a good job in his first term, health permitting one would expect that he will seek a second term. It is tough to beat an incumbent President: you cannot compete with a candidate who has Air Force One 747 as his background image. That is why his statement is puzzling: it may be less to do with what voters like — as he suggests — and more to do with not wanting a second term.
I am sure Obama (or Clinton) will bring up the issue by saying “Do you want a President who is willing and able to serve you for 8 years or someone who just wants to be President”.