Castro, 81, has not been seen in public for 16 months other than in television images after he had stomach surgery for an undisclosed illness, Reuters reported.
"My basic duty is not to cling to office, nor even more so to obstruct the rise of people much younger, but to pass on experiences and ideas whose modest value arises from the exceptional era in which I lived," he said in a letter read on Cuban state television, according to Agence France-Presse.
Castro, who took power in a 1959 revolution, appeared to hint in his letter that he would leave the country's top leadership to his brother Raul Castro, 76, who has served as interim leader since July 2006.
Cuban officials have said Fidel Castro keeps up with official business while he is recovering in an undisclosed location. Since March, his weekly opinion pieces on global affairs have been published in Cuba's newspapers.
But there has been no official indication of whether or when he would resume the presidency. Nor has there been any clear information about the state of his health during his lengthy convalescence.
In the letter, Castro also commented on Saturday's climate accord in Bali, which set a 2009 deadline for negotiations on a new treaty to tackle global warming. The United States -- the world's top emitter of greenhouse gases -- agreed to the climate pact but voiced "serious concerns."
"It is obvious the United States maneuvered to avoid isolation, although it did not signal any change in the empire's somber intentions. The great show started, and Canada and Japan joined it immediately," Castro said, according to AFP.
Castro also invoked the example of a renowned Brazilian architect who is still working at 100.
"I think like (Oscar) Niemeyer that you have to be of consequence up to the end," Castro wrote in Monday's essay, referring to lifelong communist who was honored around the world as he turned 100 on Saturday, according to the Associated Press.
Despite his physical absence from public life, Castro is up for re-election as a deputy to the National Assembly during Jan. 20 elections.
Within weeks after that balloting, the newly formed parliament is expected to select a new Council of State and its president, but there has been no official word on Castro's role. He has held the council presidency since its 1976 creation.
When the parliament re-elected Castro to his sixth term as Council of State president in March 2003, Castro said he would stay power only as long as he felt he was contributing. But he had not mentioned his future role until Monday.
The White House said Tuesday it was difficult to determine what Castro's letter actually meant, according to Reuters.
"It was an interesting letter," spokeswoman Dana Perino said. "It's hard to make out what he is saying or what he means, as is not unusual.
"And so we're just continuing to work for democracy on the island, and we believe that that day will come soon."
President Bush, in an October speech on Cuba, called the Castro government a "disgraced and dying order" and urged Cubans to push for democratic change.
Senior government officials, who no longer say that Castro is recovering and will return to office, insist that he is consulted on major policy decisions, Reuters reported. His illness last year sparked speculation about the end of one-party Communist rule in Cuba.
But most observers agree that a stable transfer of power has occurred to Defense Minister Raul Castro as acting president.
The younger Castro, who is considered to be a more practical administrator, has encouraged debate on the country's main economic problems and promised "structural changes" in agriculture to ensure Cubans have enough food.
Seven out of 10 Cubans were born after Castro's revolution and have known no other leader. Many are unsure what the future holds in store after Castro.
"We are ready, but we don't know what will come. We expect good things, nothing bad we hope," said Ana Rosa Hernandez, an usher at Havana's Yara cinema, quoted Reuters.
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