Nice! I'm trying to get to sub 20 5K this spring. Right now I'm doing 8:30 to 8:50 miles to build a base.
Start hitting the treadmill. You need a pace you can set and increase over time.
Set the incline to +1.5* to better approximate running outdoors. I do it in the morning prior to eating breakfast. 5km = 3.10 miles.
This is what I do over the cold reason as an example.
Last year it was:
Walk 3.0 mph for 3 mins, 0.15 mile (warm up)
Run 8.0 mph for 2.80 miles, 2.95 total distance
Run 10.0 mph for 0.35 miles, 3.30 total distance (3.15 miles at running pace)
Jog 6.0 mph, for 0.40 miles cool down, for 3.70 total distance
Walk 3.0 mph for 3:00 mins.
Stretch, 40 pushups.
Make sure you are strength training your legs on your off days as well as your upper body. I do squats and lunges only for leg work, don't over do it though.
This year I'm doing
Walk 3.0 mph for 3 mins, 0.15 mile
Run 9.0 mph for 2.80 miles, 2.95 total distance
Run 11.0 mph for 0.35 miles, 3.30 total distance (3.15 miles at running pace)
Jog 6.0 mph, for 0.40 miles cool down, for 3.70 total distance
Walk 3.0 mph for 3:00 mins.
Stretch, 60 pushups.
Should be around 30 mins. Good luck. It's a lot of work. Keep breathing, the last sprint is pretty tough.