"I was a bully," he told Winfrey of how he treated others who might expose him.
But Armstrong was not telling the whole story, author David Coyle, who wrote a book about doping and the Tour de France, told CNN's Anderson Cooper on Thursday night.
"A partial confession is sort of the pattern here," he said. "Maybe this is Armstrong's partial, and more will come out later."
The cyclist denied pushing teammates to dope, an assertion Coyle countered.
"Tyler Hamilton gets a phone call: Be on a plane tomorrow. We're flying to Valencia to do a blood transfusion. That's what happens," Coyle said.
Armstrong described his years of denial as "one big lie that I repeated a lot of times." He had races to win and a fairy tale image to keep up.
He reminisced on his storied past of being a hero who overcame cancer, winning the Tour repeatedly, having a happy marriage, children. "It's just this mythic, perfect story, and it isn't true," he said.
Former teammate: More needs to happen
Former cyclist Tyler Hamilton, who raced as a teammate of Armstrong from 1998 to 2001, said it was nice to hear Armstrong own up to some of his faults.
Hamilton was among those who broke from Armstrong and decried his use of performance-enhancing drugs. Armstrong at the time denied the allegations and threatened him with legal action.
"He's made the first step," Hamilton told CNN's Piers Morgan before the second interview aired. "There are many more steps to come for Lance Armstrong."
According to Hamilton, the next step for the disgraced cyclist is to testify before cycling officials about the doping network, naming names as needed.
The U.S. Anti-Doping Agency, which tests Olympic athletes for performance-enhancing drugs, similarly described the interview as a "small step in the right direction."
"If he is sincere in his desire to correct his past mistakes, he will testify under oath about the full extent of his doping activities," USADA CEO Travis Tygart said.
The International Cycling Union called it "disturbing" to see Armstrong's confessions, but it said the sport is much different today than it was 10 years ago.
"Lance Armstrong's decision finally to confront his past is an important step forward on the long road to repairing the damage that has been caused to cycling and to restoring confidence in the sport," union president Pat McQuaid said.
Years of success and defiance, then a rapid fall
After winning various legs of the Tour de France, Armstrong's sporting career ground to a halt in 1996, when he was diagnosed with cancer. He was 25.
He told Winfrey that he then developed a "ruthless and relentless" attitude that helped him survive. But he carried it with him into his sports career, "and that's bad," he said.
He returned to the cycling world, however. His breakthrough came in 1999, and he didn't stop as he reeled off seven straight wins in his sport's most prestigious race. Allegations of doping began during this time, as did Armstrong's vehement defiance.