"Now the story is so bad and so toxic, and a lot of it is true," Armstrong said.
The U.S. Anti-Doping Agency, which tests Olympic athletes for performance-enhancing drugs, said the interview was a "small step in the right direction."
But it seemed to share Coyle's skepticism about whether Armstrong was exposing the whole truth.
"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 Olympic Committee called it a sad day for the sport, and likewise called for Armstrong to "present all the evidence he has to the appropriate anti-doping authorities so that we can bring an end to this dark episode and move forward, stronger and cleaner."
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
The scandal has tarred the Livestrong cancer charity that Armstrong founded and blown his endorsement deals.
Those who suffered for speaking out now feel vindicated.
They include Betsy Andreu, wife of fellow cyclist Frankie Andreu, who said she overheard Armstrong acknowledge to a doctor treating him for cancer in 1996 that he had used performance-enhancing drugs.
"This was a guy who used to be my friend, who decimated me," Andreu told CNN's Anderson Cooper on Thursday night. "He could have come clean. He owed it to me. He owes it to the sport that he destroyed."
The former athletic icon conceded he'd let down many fans "who believed in me and supported me."
"I will spend the rest of my life ... trying to earn back trust and apologize to people."
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.
He left the sport after his last win, in 2005, only to return to the tour in 2009.
Armstrong still insists he was clean when he finished third that year, but that comeback led to his downfall.
"We wouldn't be sitting here if I didn't come back," he told Winfrey.
In 2011, Armstrong retired once more from cycling. But his fight to maintain his clean reputation continued. Federal prosecutors launched a criminal investigation, but it was dropped in February.
In April, the USADA notified Armstrong of an investigation into new doping charges. In response, the cyclist accused the organization of trying to "dredge up discredited" allegations and filed a lawsuit in federal court trying to halt the case.
The USADA found "overwhelming" evidence that Armstrong was involved in "the most sophisticated, professionalized and successful doping program."
Armstrong objected to the claim to Winfrey, saying that although it was "professional," it did not compare to doping programs in former communist East Germany.
Armstrong told Winfrey that the unraveling of his career is the second time in his life that he could not control his life's narrative -- the last time was when he had cancer