Jenner earned the title of "World's Greatest Athlete" nearly 40 years ago. The Olympic gold medal in the decathlon catapulted Jenner into the spotlight.
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Most recently, Jenner has been seen with his large family on the reality television show "Keeping Up With the Kardashians."
Now, it's time for Jenner's revelation.
"It's going to be an emotional roller coaster, but somehow I'm going to get through it," Jenner said.
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When asked, "Are you gonna be okay?" Jenner answered, "Yea. I hope I'm gonna be okay. I feel like I'm gonna be okay. 2015's gonna be quite a ride. Quite a ride."
But on Friday morning, Sawyer wouldn't say exactly what the Olympic hero is going to reveal in the two-hour special.
"Some stories should only be told by the person who lives them," she said. "And he wants to speak directly tonight, so I'm going to let him say first what he wants to say."
She did call the interview "a family love story."
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Jenner, a father of six and stepfather of four, says it's family he worries about most.
"Those are the only ones I'm concerned with," said Jenner. "I can't let myself hurt them."
Diane Sawyer says Bruce Jenner knows this is a big day for him. The Olympic champ is excited for the future and ready to go out and do some good in the world.
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PHOTOS: Bruce Jenner through the years