Jog lightly around the track one time with as little effort as possible as you start training to run longer and stronger. You should feel as fi you barely went from walk to run. This will get your body warmed up. Make sure you're breathing correctly: in through the nose and out through the mouth in a rhythmic pattern. Some runners like to inhale for a count of three and exhale for two.
Jog slowly as you begin the second lap. Hold your arms loose and keep your step light.
Pick up the pace when you reach the first turn, doubling your step speed. Keep building speed until you reach the end of the curve that begins the other long side of the track.
Sprint when you reach the start of the long side of the track. Run as fast as you can.
Slow down when you reach the start of the next curve. Return to the double-time jogging speed you had before you sprinted. Maintain that speed until you reach the end of the curve to the next long side of the track. From there, jog slowly.
Jog slowly until you reach the next curve, then increase your speed as you did in Step 3. At the end of that curve, sprint. When you reach the start of the next curve, slow down to double-time speed, slow jog, and then eventually walk until you reach your original starting point.
Do this speed workout every other day. After the first week, increase your jog/double time/sprint laps by one. At the end of the second week, time yourself to see how long it takes to run four laps at a normal pace. Increase the exercise routine by an extra lap each week. Eventually you will see your four-lap time decrease.