Yes, it's great that these guys are doing so well in their 30's, but I actually wish that some new blood would come through and win Grand Slams and give us some new players to get excited about. I think this AO shows just how competitive both men's and women's tennis is now. No-one would have had Venus in the final. On men's side the top 8 are all out and most of them did not even get to the QFs. Makes it all the more exciting in my view, and certainly doesn't make Andy, Novak etc "failures" for not getting to the final.
Fed has been one of the best technicians in the game. He's recovering from injury and hasn't played like his old self of late, but I doubt he stopped believing in himself. Losing his confidence doesn't seem to be a problem for him, not that he comes off as conceited, but I'm sure he wasn't surprised by his win.
Federer was probably the least surprised in his victory because Federer is the one who knows his own ability. He is the one who has been training with himself day in and day out most of his life. That means he knows what he is capable of more than anyone else.
Otherwise, he would not have put in the hours of training and preparation for the event that he had in order to stage a comeback. He had a clear mission which he believed he could accomplish, or else he would not have bothered such a risky attempt at a comeback. He must have believed he could win in order to compete and it is doubtful that his victory came to him as a surprise.