Now that spring is here, are you spending more time running around outside? Have you discovered that you don't feel quite as strong and energetic as you'd like to? Maybe you're "out of shape" after a long winter spent indoors.

If you're feeling out of shape, you're not alone. The President's Council on Physical Fitness and Sports recently reported on the results of its School Population Fitness Study. For the study, fitness experts examined 18,857 children aged 6 to 17. They discovered that many of the kids performed poorly at some pretty basic activities like running, jumping, flexibility and strength.

May 1986 is National Physical Fitness and Sports Month. As part of the celebration, the American Academy of Family Physicians, a group of doctors who treat children and families, and the President's Council on Physical Fitness got together to publish a coloring book called "I'm a Fit Kid." It stars the Hallmark character Rainbow Brite. The book is designed for kids aged 3 to 8. About 1.5 million of these coloring books will be distributed free through doctors' offices and schools during May.

The coloring book contains some good ideas about how to keep fit. The book reminds kids to exercise regularly, and outlines a daily workout plan, including these suggestions:

Always warm up with exercises like arm circles, knee bends and walking in place. Then carefully stretch all your muscles.

Build your muscles by doing sit-ups, push-ups, leg-lifts, climbing activities and similiar activities.

Strengthen your heart continuously for 30 minutes by doing activities like running, bicycling, jumping rope or swimming. You may have to start with 10 minutes and work your way up to 30 minutes.

Cool down after your workout by carefully stretching all your muscles again.

Play games and sports as often as you can with your friends and family.

Many people find that taking part in a regular activity like an exercise class, a softball team, a ballet group or a soccer club helps them stay in good shape. In most of these groups, kids exercise with other kids. But in one program in Maryland, kids and older people exercise together. The class is called "Fitness Fun Together." The Bethesda-Chevy Chase YMCA, the Ayrlawn Child Care Center and the Montgomery County Intergenerational Committee cooperated to design the class.

The exercise class pairs 4-year-olds with elderly people. Together, the partners go through a 45-minute session of stretching, energetic movement to music, and slow-motion exercises. In one of the classes, the kids and the older people danced around holding long blue scarves. In another one, they played with pinwheels while learning about the importance of breathing correctly during exercise. The kids had fun, and so did the grown-ups. One woman especially enjoyed the class because she lives far away from her family and doesn't get to see her grandchildren very often. She liked having a chance to spend time with kids.

The class at the YMCA in Maryland was an experiment. After it ended the teacher and others who had been involved in organizing "Fitness Fun Together" decided that the class had been so successful that they would offer it again.

Keeping fit is fun -- and it's a good health habit. Experts think that fit kids may feel better about themselves, do better in school and catch fewer sicknesses than kids who are in poor shape. Your body just works better and feels better when you're getting regular exercise.

As the weather warms up, and summer vacation gets closer, you might think about planning a fitness program for yourself. Your plan could be something simple, like taking a daily walk with your dog. It could be more complicated, like getting involved in Little League, taking a dance class or joining a soccer team. If you start now, you'll be an expert by the time you're grown up. Maybe you should get your parents -- and even your grandparents or other older people you know -- involved in your fitness plan. It worked at "Fitness Fun Together." It might work in your family or your community, too. Tips for Parents

How good is your child's school physical fitness program? The President's Council on Physical Fitness and Sports, Washington, D.C. 20001, says that such programs should:

Offer at least one period per day of vigorous aerobic activity, not just play.

Teach kids sports they will be able to do throughout their lives, such as aerobic dancing, swimming, bicycling, and running, in addition to team activities.

Give a screening test to identify children who lack cardiovascular endurance, flexibility, strength and coordination.

Put as much or more emphasis on physical education for youngsters who are overweight, unfit or unskilled.

Offer physical education to children with special problems such as physical or mental handicaps.