The Spaniard finished just three points shy of Sebastian Vettel despite having an inferior car for much of the season.
Speaking at a Santander media event in Spain, Alonso doubted that his 'perfect year' could be repeated.
"You just have the feeling that some years you have done a good job and others when you feel there was something missing or something you wanted to change or want to improve for next year," he said.
"I think it's been a perfect year. It's going to be nearly impossible to repeat that in my career."
Alonso also believes that Red Bull will start 2013 as favourites, with his Ferrari team needing to close a gap of around three-quarters of a second a lap.
"We were some 2.5 seconds off the pace in the Jerez test when the car made its debut, so it's nearly impossible that it will be worse, and so I expect a better start," Alonso said, a reference to the troubled beginnings of Ferrari's F2012.
"But there's no doubt that Red Bull will be favourite at the start of the season. They finished the season with a pretty strong dominance and rules are more or less the same."
"They had a seven, eight-tenth advantage [at the end of the season] so we need to close that gap in two months, which is not easy."
There's little doubt that Fernando Alonso's 2012 season was extraordinary, with only his mistake in Japan costing him dearly. Drivers usually reach their prime in their early thirties and considering that Alonso has been improving with every year he's been at Ferrari, you'd be a fool to think he won't be up there in 2013. After all, it isn't as if he's suddenly appeared from nowhere; Alonso has been driving strongly since the start of 2005, save for a dip in form in the first half of 2008.
Regarding the car, the rules stay fairly stable and there is a lot of unlocked potential in the F2012. The alterations to the DRS regulations could boost Ferrari, as instability under braking when the DRS re-attaches is understood to be the root of their qualifying issues. If Ferrari can deliver Alonso a strong car - not necessarily the best, think back to 2010 - for most of the season, he'll undoubtedly be in contention for that elusive third title.