It depends on different things.
How old are you and how long have you been running? Young runners who just started have greater potential to improve than old runners who run for several years.
Do you have an experienced coach, who plans your training and a running group which helps you to stay motivated, or do you do everything on your own?
Do you do mile specific training? That means workouts like 10 x 400 at mile pace (start with 80 sec and try to improve to 74 s), or 15 x 200 between 800 m pace and mile pace (maybe 37 s now, improve to 34 s gradually).
Do you have good basic speed? For a sub 5 mile you need a 2:15 800 m, but with a 2:10 it would be even easier. That requires sub 60, maybe even 57 s 400 m speed. In the end you can break it down to a 100 in 13 s. For to improve your speed do some short sprints on the track, 50-150 m at full speed with long recovery (up to 5 min).
Don't neglect to do some strength training twice a week. Core stability exercises and maybe even some deadlifts and squats. You don't want to develop a massive body builder type, but a bit of general athletic work will help to improve your running, too.