Through daily exercise, Fidel "has recovered a lot of weight and muscle mass", Raul said to voters in Fidel's district in the eastern city of Santiago.
Raul said Fidel had asked him to visit voters because he was unable to.
In remarks carried on Cuban state television on Monday evening, Raul said his brother "has more time, he's reading more than ever. He's meditating more than ever and writing almost more than ever".
"His powerful mind is healthier," the acting president said.
Democratisation
Last week, Fidel, 81, suggested he would not cling to power forever, nor stand in the way of a younger generation.
He hinted at his political future for the first time since emergency intestinal surgery forced him to cede power to a "provisional" government headed by Raul in July 2006.
Speaking of Cuba's electoral system, Raul noted that US democracy pits two identical parties against one another, and joked that a choice between a Republican and Democrat is like choosing between himself and his brother Fidel.
"We could say in Cuba we have two parties: one led by Fidel and one led by Raul, what would be the difference?" he asked.
"That's the same thing that happens in the United States ... both are the same. Fidel is a little taller than me, he has a beard and I don't."
Raul scoffed at the notion Cuba needs to be more like the US, but he also acknowledged that the island's communist government has its flaws, saying that Cuba's "system has to become more democratised".
"If we only have one party that represents the interests of the people, where we can have differences, we should have them," he said.
"Not class clashes, but it's good to have differences," he said.