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Cyclist Lance Armstrong, right, admitted to Oprah Winfrey (left) in an interview on Monday that he used performance enhancing drugs during his illustrious career that saw him win seven Tour de France titles.

Oprah Winfrey says she was "mesmerized and riveted" by her interview on Monday with Lance Armstrong in which the disgraced cyclist admitted to using performance enhancing drugs.

Winfrey appeared on CBS Tuesday morning to discuss her interview with Armstrong, which will air over two nights on Thursday and Friday on the Oprah Winfrey Network.

Armstrong admits to doping Alessandro Trovati / AP A controversial career   Lance Armstrong's 7-year reign over the Tour as historic as his fall from grace over drug use. Oprah finally gets Lance to crack emotionally Analysis: Armstrong left too much unanswered Armstrong's enemies find vindication, sadness J.J. Abrams to produce Lance Armstrong biopic Analysis: Will Lance help clean up his mess? Ex-teammate: Lance is 'worthy of redemption' Federer, Serena say Lance let athletes down Schleck believes Lance clean after comeback Lifetime ban is "death penalty," says Armstrong Triathlete: What's Armstrong's motive for talking? CSN's Ratto: Nobody's buying what Lance is selling

Winfrey described the interview as "forthcoming" but chose not to reveal many details.

“He did not come clean in the manner that I expected," she said. "It was surprising to me."

Armstrong, 41, spent more than a decade denying that he used performance enhancing drugs. On Monday, he personally apologized to staff members of the Livestrong charity that he founded.

In a statement to NBCNews.com, Livestrong director of communications & external affairs Rae Bazzarre confirmed a conversation took place.

"Lance came to the Livestrong Foundation's headquarters today for a private conversation with our staff and offered a sincere and heartfelt apology for the stress they've endured because of him and urged them to keep up their great work fighting for people affected by cancer."

The interview was to be broadcast on Thursday, but Winfrey said it will now run in two parts over two nights because there is so much material. She added that it was too big of a story to “leave half of this on the cutting room floor.”

Armstrong won every Tour from 1999 to 2005, but each of those titles was stripped last year as the U.S. Anti-Doping Agency released a massive report built around the testimony of former teammates. USADA accused Armstrong of masterminding a long-running and sophisticated doping operation on his teams.

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