Beginning of school year/cooler weather good time for 60 min. exercise Beginning of school year/cooler weather good time for 60 min. exercise. Published: Aug. 29, 2013 at 10:42 PM DALLAS, Aug. 29 (UPI) -- The beginning of the school year and/or cooler weather is a good time to incorporate the recommended 60 minutes of exercise most days, a U.S. expert says. Lona Sandon of the University of Texas Southwestern Medical Center in Dallas said adults should get 60 minutes of moderate to vigorous physical activity most days. She recommends those getting serious about exercise to: Mix up your workouts to include both cardiovascular and muscle training exercises for a whole body fitness improvement plan. Cardiovascular exercise helps keep your heart healthy while muscle training exercise can improve posture and strength. Try interval training to improve cardiovascular fitness and burn up calories. Interval training requires you to alternate bouts of high intensity with lower intensity during your workout. Lift heavy enough to make a difference to build muscle. When strength training, the weight should be heavy enough to tire out the muscle with about 12 repetitions. Allow muscles at least one rest day between hard strength building workouts. Always start with a warm-up. A few minutes of light aerobic activity followed by gently stretching the major muscle groups you are about to use will get the body temperature up and help prepare your body for the workout. It can also help prevent injuries. Cool down with 10 minutes of stretching. Being physically fit includes being flexible. After your workout, your muscles will be warm and flexible. This is a good time to stretch them out which may help prevent muscle soreness and injury.