"I am not physically able to speak directly to the citizens of the municipality where I was nominated for our elections," the ailing 81-year-old wrote in an essay published Wednesday by state news media.
Castro's latest writing focused on blasting U.S. President George W. Bush, but included references to the Cuban leader's health.
However, a new video released Wednesday on Cuban TV shows a more animated, if still thin, Castro, reports CBS News producer Portia Siegelbaum from Havana. His color appears better than in previous video. And while only hear his voice is heard in one short statement ("I've felt very well"), it is spoken more clearly and firmly than in past video released by government.
Castro's essay was published on the front pages of state-run newspapers a day afterwith Brazilian President Brazilian President Luiz Inacio Lula da Silva, who said Castro appeared healthy enough to return to politics.
"I think Fidel is ready to take over his historic political role in this globalized world, in humanity," Silva told reporters as he left Cuba late Tuesday. He did not suggest what role that might be.
Castro, however, expressed frustration in his essay: "I do what I can: I write. For me, this is a new experience: writing is not the same as speaking."
He has not been seen in public since July 2006, when emergency intestinal surgery forced Castro to cede power to a provisional government headed by his brother Raul, five years his junior.
Raul Castro addressed a crowd of voters on Dec. 24 in the brothers' home district in the eastern city of Santiago, saying he was filling in for Fidel.
Wednesday, however, was the first time the older Castro has acknowledged he is not well enough to campaign for himself - though there was no sign he would pull out of the election.
Re-election to the legislature, or National Assembly, is a necessary step if Fidel Castro is to continue as head of the Council of State, Cuba's supreme governing body.
Voters on Sunday will be given lists of candidates for parliament chosen by municipal councils. They can vote for some or all of the candidates.
Castro looked frail but upbeat and even playful in official photos of his meeting with Silva.
Wearing a track suit and tennis shoes - which have replaced olive-green fatigues as his standard uniform - Castro is seen seated and grinning, his beard well-trimmed and his hair combed as he talks with Brazil's president At one point, he even pretends to snap pictures with a small camera.
"My feeling is that Fidel is in very good health, that he's as lucid as he's ever been," Silva said, adding that politicians were like athletes who need to stay active.
Cuba's government has not given details about Castro's illness or where he is being treated. The photos with Silva were the first images of Castro released since an October meeting with his socialist ally, Venezuelan President Hugo Chavez.
"He looks really recuperated, with a lot of energy," said Havana resident Manuel Puerta, looking at a photo of Castro on the front page of the Communist Party newspaper Granma. "Taking into account his long illness, I think he looks very good."
Castro suggested in a letter last month he would not cling to power forever or stand in the way of a younger generation of leaders. But he also mentioned the example of a Brazilian architect who is still working at 100 and has not said when - or even if - he will permanently step aside.