Winfrey discussed the "no-holds barred" interview on "CBS This Morning" Tuesday.
The interview, conducted in Austin, Texas, Monday, was set for broadcast on Winfrey's network Thursday. After the interview, Winfrey tweeted, "Just wrapped with @lancearmstrong More than 2 1/2 hours . He came READY!"
It was a pivot for Armstrong following years of public statements, interviews and court battles in which the former Tour de France winner denied doping and zealously protected his reputation.
Hours earlier, Armstrong appeared at Livestrong, the cancer charity he founded, and apologized.
The cyclist lost all seven of his Tour de France titles following a U.S. Anti-Doping Agency report that portrayed him as a ruthless competitor, willing to go to any lengths to win the prestigious race.
In a recent interview USADA chief executive Travis Tygart described Armstrong and his team of doctors, coaches and riders as similar to a "mafia" that kept their secret for years and intimidated riders into silently following their illegal methods.