"I mean, there's so much more to do here in the present ...," Obama told "Good Morning America" Robin Roberts in an exclusive interview that aired on the show today. "I focus on what's before me, right on the work that I can do today."
"Whenever it's time to campaign and they tap me on the shoulder, I'll be right there. But until then, we've got some great initiatives...," she said. "And I'm rolling up my sleeves to get that work done, and we'll cross that other bridge when we get there."
Roberts' interview with Obama took place at Fort Jackson, the U.S. Army training post outside Columbia, S.C. The first lady visited the base on Thursday to get a first-hand look at how the Army is tackling the problem of obesity among service members.
Last February, she launched "Let's Move!," a campaign that seeks to end childhood obesity within a generation. Obesity is of particular concern for the military, she said, pointing out that some top Army officials have said young recruits are unable to train because they are either overweight or malnourished.
"It is not an overstatement to say that childhood obesity and our need for physical education and nutrition education is a national security issue," she told Roberts in a wide-ranging interview that also touched on the Tucson shootings, support for military families, the government's role in people's personal nutritional choices, and criticism of her fashion choices.
The first lady said the nation had made strides in nutrition legislation and public education, adding that the second year of the "Let's Move!" initiative would bring enhanced efforts to make more public schools healthier, as well as outreach to faith-based and community organizations.
First Lady: Better Habits Mean Savings for Nation
The point is to "make nutrition fun again," she said.
The reliance upon heavily processed food, fast food and sweetened drinks is taking its toll. Add to that changes in society -- where increased computer usage and more TV time mean children aren't getting outside to play as much as they used to -- and the situation grows more serious.
"If we can make our society healthier, if we can make this next generation of kids healthier, we can teach them how to eat, if we can get them moving just a little bit...," she said. "If we can get the military to use its resources to change the way it feeds our troops, then we'll see progress."
Better nutritional habits that last over a lifetime will mean "billions of dollars" in savings to the nation, she noted.
Asked about critics who've said the government has no role in people's nutritional choices, the first lady said the "Let's Move!" program "has never been about the government telling people what to do. But what it is about is giving parents information -- and clear information -- so that they can make good choices."
The first lady also spoke about increased support for military families in their daily lives. Earlier this week, President Barack Obama announced the new set of initiatives that will emphasize military children's education, military spouses' educations and careers, increased child care and quality of life improvement.
"You don't have to be a member of a military family or live near a base to help and reach out," the first lady said. "We can all do it in our own way. And one of the things I just tell people is to do what you do best."
Volunteering, offering professional services at no cost or hiring a veteran or military spouse were ways to help those who serve the nation's military, she said.
Tucson Aftermath Showed Nation's Spirit
The first lady said the nation's response in the aftermath of the Tucson shootings was a clear demonstration of America's true spirit.
The Jan. 8 shooting rampage left six people dead and at least 12 others wounded.
Among the wounded were U.S. Rep. Gabrielle Giffords, who police say was the intended target of alleged shooter Jared Loughner. The dead include U.S. District Judge John Roll and 9-year-old Christina-Taylor Green.
Christina-Taylor's death demonstrated "just how fragile life is, and just how important it is for you to ... hold the ones you love just a little bit tighter, and value the things that are good in others. Not to take those things for granted. That's what times like these teach us," she said.
"Even in our deepest pain, our most horrific moments, we come together, because that community stepped up and supported Gabby Giffords and all the other families ... they were there for them. And the spirit of that is something that we, you know, we need to embrace and be reminded that that's who we are as a nation," the first lady said.
"That gives me the energy to do the next hard thing, because I know who I'm working for. I'm working for my country, you know?"
Living her life in the public eye brings its own challenges.
At the Jan. 19 state dinner in honor of visiting Chinese President Hu Jintao, the first lady's choice of dress designer got people talking.
Designer Oscar de la Renta said the first lady should have worn a gown from an American designer, rather than the show-stopping red gown from British designer Alexander McQueen.
The first lady has an answer for her critics.
"Look, women, wear what you love. That's all I can say. That's my motto. I wear what I like because ... I gotta be in the dress, so," she said, adding that it was a much better use of her time for her to focus on childhood health and rallying support for military families.
Nelson Mandela's Legacy
She also spoke highly of the legacy of Nelson Mandela, the legendary South African anti-apartheid activist. Mandela has been hospitalized and concerns about the aging statesman's health are mounting.
"What do you say about someone who's changed the face of the world? Nelson Mandela is, as so many of our heroes ... paved the way for people like me and my husband and so many other who are going to continue to look up to his legacy," she said. "So we are grateful to have been alive to see the impact of his life."
Throughout his life, Mandela has continued to champion patience and understanding.
"You know, not holding on to the past, and moving towards the future. It's still relevant today ... all over the world, these are lessons that we're still working on. But he's given us a life that has given us some guidance, it's given us a little road map about how to treat one another."