Before you launch into anaerobic speed sessions, warm
up with 15 minutes of easy
jogging and then stretch. Jog for another five minutes. Always cool down and stretch after a speed session.
Run 200m intervals. Condition your body to run anaerobically and your muscles to react explosively with hard running; “hard” means a pace slightly faster than your current 400m pace, not an all-out sprint. If you run 400m in 80 seconds, then you should run 200m intervals in 38 seconds.