If you're sitting down while you read this, stand up.

Inactivity continues to plague adults and children, who spend an increasing amount of time at computer screens, in front of TV sets and commuting long distances to work and school.

Even in their off hours, Americans are often sedentary. Less than a third of adults engage in regular leisure-time activity, according to the federal Centers for Disease Control and Prevention. Adolescents don't do much better. Just 25 percent of high school students are moderately active for 30 minutes per day, according to acting Surgeon General Steven Galson. "And that's only half the time recommended. We've got to do better than that."

To help reverse this sedentary trend, the President's Council on Physical Fitness and Sports has tapped New York Giants quarterback and Super Bowl MVP Eli Manning to kick off the National President's Challenge -- encouraging Americans to be more active during the next several weeks.

How active? Thirty minutes per day for adults at least five days a week; 60 minutes per day for ages 6 to 17, also at least five days weekly. Registration runs until April 3 online at www.presidentschallenge.org. That's where you can record your activity, set goals and see how you're doing with others taking the challenge. The challenge runs to May 15.

For one and all

Enter solo, or team with family members, co-workers or neighbors in groups that can also compete against each other. Watch the numbers tick up in real time as additional participants register. As I write this column, more than 35,000 people have registered.

No need to just go to the gym either. The President's Challenge site lists more than 100 physical activities that can count toward the daily goal, from gardening to walking the dog.

"This challenge is for everybody, whether they are de-conditioned and sedentary, all the way up to the super athletes," said Melissa Johnson, executive director of the President's Council. "We believe that there is an activity for everyone to enjoy."

Studies show "that those who engage in anything for six weeks are more likely to make it part of their lifestyle and a habit," Johnson said. "Our hope is that people will continue being active once the challenge ends because they will realize how good they feel."

Finding the time

Fitting in regular exercise can be a challenge even to some of the well-known athletes who are members of the President's Council. Once their athletic careers end, many say that they struggle to incorporate regular workouts into busy lives.

"It becomes increasingly difficult to get time in for myself in light of the demands from work and family," said council member Dot Richardson, a former Olympic softball player. So Richardson walks 30 minutes daily at home on a treadmill and takes her dogs for daily jogs. "It becomes a better workout for me and the dogs by taking the walk to a jogging level," as she notes in a blog entry on the President's Challenge website.

Taking the stairs instead of the elevator is one way Olympic gold-medal gymnast Mary Lou Retton keeps active now that she has a family of her own. "Before I know it, I have accumulated 30 minutes of cardio without much effort," she notes in a blog item on the site.

And if you are still thinking of excuses on why you can't be more active, take inspiration from council member Kirk Bauer. He's an above-the-knee amputee, who just turned 60 and is executive director of Disabled Sports USA. In addition to skiing, bicycling, walking and swimming, he also water skis and plays golf.

"To get my exercise at work, I walk nearly every day to a local grocery store to get lunch. The round trip is about 15 minutes," he notes on the website. At home, Bauer rides a stationary bike and then does 100 pushups and situps, 20 at a time.

As he notes, "It is important that no matter what your disability, you can be active."

So how about it? I've already registered for the National President's Challenge. I hope you will, too, and I'd welcome thoughts about your progress at the Lean Plate Club discussion group www.leanplateclub.com/group.

You can subscribe to the free Lean Plate Club e-mail newsletter at www.leanplateclub.com. Sally Squires is a writer for the Washington Post.