Whenever I'm out and about, people ask me the secret to keeping my body so fit. (OK, it only happened once, and I think the guy was trying to sell me something.)It's so easy, I'm stunned more men don't do it. I don't go to a gym. I eat full meals. I'm not blessed with particularly great genetics. The secret is having children. Trying to keep up with my youngsters is all it takes to keep me healthy.You can tell it's working every morning. You'll feel a twinge in your neck or a tweak in your back and think back to the day before. You'll say to yourself, “All I did yesterday was play with the girls.” It all starts with the exercise regimen. In the early stages of the program, I call it “body by baby.” Constantly lifting and carrying a baby carrier seat will build up instant strength. Over time, you work on flexibility and maintaining muscle control as you place a child in a crib, careful to keep her from waking. You'll also learn your body only needs one or two solid hours of sleep instead of that crazy eight you had as a single man. Like any good workout routine, it gets more difficult over time. The second stage is “father fitness.” Every child has a little daredevil in her, and that daredevil becomes your resistance weight. For me, this stage tested my arm and shoulder strength, which were target areas for me going into the program.In no time flat, my weakness became a strength. I can extend my monkey arms outward and have a toddler grab onto it, lifting the child off the floor. For an advanced workout, try tickling your kid with your other arm without dropping her.I can also toss a 40-pound girl straight into the air and catch her. You know you'll do the exercise properly, as your child's smile motivates you to execute it perfectly.Lately, we've moved into advanced flexibility training. I lift the child from the floor as if to carry her, and then I continue lifting the child over my head until her feet hang over my shoulders, dangling her head downward. You can tell it works when you feel that burn in your shoulder and hear that giggle behind your back.This isn't just a workout for your arms, though. Your legs will get plenty of attention too. You'll start with squats, as you sit toward the floor to talk to your kids and the constant jumping up from the floor. Eventually you'll advance to the heavy weights on your feet. Sooner or later, every child will try this one on you as you try to leave for work. They'll bear hug your legs to try to keep you from leaving. Just try to walk through the door, lifting the child with each step. Unlike many exercises, this one doesn't get easier with more practice. As you build leg strength, children gain weight, resistance and will.“Sleeping beauty” is another challenging exercise. This is when you carry a slumbering child up a flight of steps after she falls asleep on a long car ride home from visiting relatives. Lately, there's a midnight variation to this exercise, running each child down two flights of stairs when inclement weather calls for a surprise trip into the basement.You'll also build speed and side-to-side movement. The first time your child sprints away from you down your street or tries to hide in a store, you'll find out just how fast and agile you can be. That parental anger of a misbehaving child gives you just the boost you need to get moving in a hurry.No health plan works on exercise alone. You also need to try the “dad diet.” Some diets claim you can eat what you like. In the dad diet, you eat what's in front of you, even if it didn't start out as yours. If your child doesn't like her hamburger, you take hers and let her enjoy yours. You'll note I mentioned hamburger and fries. That's because you'll eat a lot of those, along with chicken nuggets, hot dogs and peanut butter and jelly sandwiches, when you have children. You might get bored with the same foods every day, but your kids never tire of them. Before long, you'll forget what your favorite foods are, yet you'll know your children's. When you see their pure joy when they eat their favorite foods, you won't mind.It's hard to argue with the program's results. Sure, I'm 30 pound over my ideal weight, and I don't look particularly buff. These simple moments spent with the children create a healthy emotional lifestyle for both of us. With these intrinsic rewards of this parental program, I've never felt better in my life.