Today I went to the track and ran 14 x 300 meters (4200 meters fast running). My times descended from 1:06 to 0:56 or 6 minute mile pace to 5 minute mile pace. My heart rate maxed out at 160 bpm after running 300 meters in 1:00 or less. Total time for the workout was 29 minutes. I took a little over a minute rest after each repeat or until my heart rate dropped to about 124 bpm. My overall average heart rate was 147 bpm.