Since you’re presumably young, you should use almost all of your limited time getting aerobic exercise like running, biking, or brisk walking, said William J. Evans, director of the Nutrition, Metabolism, and Exercise Laboratory at the University of Arkansas for Medical Sciences. That’s because “regular aerobic exercise increases life expectancy by decreasing the risk of a host of chronic diseases,” he added in an e-mail.

But if you were over 40 and especially if you were over 60, “an increasing amount of the three hours per week should be spent in strength training,” he said. Research shows that strength training is important for offsetting the age-related weakness that can limit normal activities, like climbing stairs. So if you’re between 30 and 40, you should aim for 120 minutes a week of aerobic exercise, plus 60 minutes (that is, a 30-minute session, twice a week) of weight lifting to build strength. After 40, you should divide your 180 minutes equally – 90 minutes for aerobics, 90 for strength training.

Even though your thrice-weekly program is less ideal than exercising at moderate intensity at least five days a week, you’re probably doing “enough to get a healthful dose” of exercise, said Steven Blair, professor of exercise science and epidemiology at the University of South Carolina. And take heart, he added. “Carrying kids is resistance training” because, like lifting weights in a gym, it builds muscle.

“The best exercise,” added Blair, “is what you will do, not what you can do.”