Earlier this week, just days before she gave an impassioned and deeply personal speech about the importance of girls' secondary education worldwide, First Lady Michelle Obama penned a poignant essay in the Atlantic on the same topic. In it, she wrote about the Let Girls Learn initiative she and President Obama launched earlier this year to address the problem of the 62 million girls worldwide who are not in school.

She closed the op-ed with this declaration: "As a first lady, a mother, and a human being, I cannot walk away from these girls, and I plan to keep raising my voice on their behalf for the rest of my life."

It wasn't the first time Obama had begun to hint at what might fill her time once her years in the White House are over. In October, she made similar remarks when she addressed Fortune Magazine's Most Powerful Women Summit in Washington, speaking about the same initiative on girls' education. "I might be in my final stretch of my time as first lady — I’m not counting down or anything like that — but I plan to continue this work for the rest of my life. I’m in this for the long haul."

Working to improve girls' education is not all she'll be doing. For instance, while in Qatar on Wednesday, she told military troops that she would also continue addressing the needs of veterans and their families: "That's not just my vow to you as first lady. It's my vow to you for the rest of my life."

Back in February, she said at a health-care summit that she does not plan to slow down on her healthy living campaign, either. "I do not have a one- or two-year horizon for this work," she said. "I have a rest-of-my-life horizon." And last year, she told graduates of a D.C. college program that her focus on getting young people into college is not just something she'll be doing "for the rest of my time as first lady, but for the rest of my life.”

If the phrase sounds redundant, it could be because Obama is all too aware of how much time she has left once they leave 1600 Pennsylvania Avenue. She will have just barely turned 53 when this country inaugurates the next president — her birthday is Jan. 17 — making her the youngest departing first lady since Jacqueline Kennedy. (Hillary Clinton was also 53 at the time of George W. Bush's inauguration, but her October birthday was a few months prior.)

While her favorability ratings haven't matched those of Laura or Barbara Bush, they have consistently beaten her husband's. That's led some to speculate whether she might run for office herself — a possibility that she has categorically, emphatically denied.

Nonetheless, given her popularity, her age, and her career as a lawyer and hospital executive, one thing that's clear is Obama's post-White House years are sure to invite more than the usual scrutiny. What also seems clear is that she plans to keep focusing on the four primary issues she's spotlighted as first lady: childhood obesity, military families and veterans, post-secondary education, and advanced schooling for girls worldwide.

There's no reason she can't continue working on them all. But there is something about the girls' education campaign that seems like it has the potential to rise above the rest in the years to come.

It is the newest initiative to be launched, for one, and thus may have the most work left to be done. It has a global reach that would seem a fitting next step for someone who's already had a front seat to the presidency of the United States. And it's an issue that has economic and social impacts that have enormous reach, even beyond the education of young girls.

Meanwhile, Obama has shared how personally resonant the issue is to her, both as the mother of two daughters and as a young girl once herself. "That story about the transformative power of education?" she said in the speech in Doha. "That's my story. That's my family's story." 

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