I can run top speed for like 1 minute of
Lay down on your back and lift your legs up about an inch above the ground. Hold this as long as you can.
Use a treadmill and set it to target heartrate. (180-170) for young people and holdit for a safe amount of time. 20 mins, then 30, and move up. Continuing this will let you hold it for longer and you will find you can go faster with the same heart rate. Also use some incline, it will keep you at safe speeds and add a little more work and less sprint.
Warrior ropes, squats, running, jogging, jump rope. Do cardio for as long as you can and everyday try more