Much of Caitlyn Jenner’s transition has been up for public consumption, but the Olympian managed the seemingly impossible: She underwent gender reassignment surgery in January of this year, and nobody suspected a thing until she revealed it to the world herself.

Jenner, 67, shares and bares all in her upcoming memoir, The Secrets of My Life, due out April 25 — and it’s there that she talks about the experience for what, she writes, will be the first and last time.

“I just want to have all the right parts. I am also tired of tucking the damn thing in all the time,” she explains, adding that she was excited to realize she’d be living authentically for the first time in her life. “The surgery was a success, and I feel not only wonderful, but liberated.”

She continues, “I am going to have an enthusiasm for life that I have not had in 39 years since the Olympics, almost two thirds of my life.”

In the book excerpt obtained by RadarOnline.com, Jenner reveals she finally made the full transition after being annoyed by fans asking intrusive questions about her genitals. But that doesn’t mean fans have the right to continue to pester her about it. As she says in the book, “You want to know, so now you know. Which is why this is the first time, and the last time, I will ever speak of it.”

Jenner’s upcoming tome netted a huge advance — $4 million, to be exact — and she worked on it with Friday Night Lights author Buzz Bissinger, who also wrote the Vanity Fair profile that introduced Caitlyn to the world. Jenner’s life has certainly exploded since she revealed her transition to Diane Sawyer on 20/20 just two years ago this month.

“For all intents and purposes, I am a woman,” she told Sawyer. “My brain is much more female than it is male. That’s what my soul is. Bruce lives a lie. She is not a lie. I can’t do it anymore.”

Which is why gender reassignment was a natural next step. As Jenner continued to embrace her new form, she found she had no use for anything that was associated with her old self. As she writes about the choice to undergo surgery, “So why even consider it? Because it’s just a penis. It has no special gifts or use for me other than what I have said before, the ability to take a whiz in the woods.”

Of course, the more public aspect of Jenner’s transition from Bruce to Caitlyn continued long after her interview with Sawyer, including her groundbreaking Vanity Fair cover story and photo shoot in June 2015, which Jenner likened to being better than winning a gold medal. She became active on Twitter soon afterward, and now boasts nearly 4 million followers. That July marked the debut of her E! reality show, I Am Cait, which was canceled after two seasons. Jenner officially changed her name in September 2015.

Previously, Jenner was reluctant to discuss gender reassignment — even going so far as to shoot down any comments on it in her Sports Illustrated appearance last June. “It’s nobody’s business whether I want to do that to my body,” she told SI. But she did admit that “Little Caitlyn” had been inside her since her youth, and it took a long time for her to come to terms with it.

“Sometimes she raised her cute little head more than others. I was female inside, but I wasn’t an effeminate male. So I could hide easily in the male world,” Jenner said of that period of her life. “My life was distraction after distraction after distraction. Being a macho male was a way for me to try to convince myself that the woman living inside of me really isn’t living inside me.

“It disgusted me,” Jenner said of her male body. “I was big and thick and masculine. The rest of the world thought it was this Greek god kind of body. I hated it. But it’s what I was given, so I just tried to do the best I could with it.”

But now that she’s undergone gender reassignment surgery, Jenner can truly live freely as her authentic self.