I'm a short distance sprinter?

The only way is working on your endurance and doing more 400 repeats training your body for endurance. The mile race is really about 60% aerobic and 40% anaerobic so you can improve to under 6 minute pace if you wanted to and probably in the 5 minute range with some serious training. Some sprinters won't be able run well over 400 meters since they specialize and won't train for longer events. If you wanted to be serious you should run XC in the fall do a few 5K races and practice longer intervals such as 400 meters X 8 at mile pace with rests to 120 heart beats or 800 meters X 4 at 2 mile race pace. Good luck!