The 30 minute barrier can be tough to break, and you’ll need to hold a 9:40 per mile pace throughout the race. For most people, running a 30 minute 5K is a good, ambitious goal. During the first mile, many runners feel excited and fueled by adrenaline and the energy of the race. The second mile can be more daunting, as fatigue begins to set in.
During harder or faster runs, you should breathe deeply but comfortably. For most runners, this means that they breathe in through both the nose and mouth to make sure they get enough oxygen. With each breath, try to breathe deeply from your belly, not your chest. This can help prevent side stitches.
The miler is running really fast when they’re on the track and really slow when they’re not. US middle-distance runner Brenda Martinez, who has PBs of 1:57.91 for 800m and 4:00.94 for 1500m

Still keep running high mileage, but replace 1 or 2 of your slow runs each week with a tempo run (~87-90% of desired 5k pace for 20-40 mins) or a speed session where you do intervals. If you don't have a track then you can do these intervals on the road, but it helps to go to a track and do something like 8x400 @ desired 5k pace w/90 seconds rest.

To run a 5k in 20 minutes, you must run at an average speed of 9.3 miles per hour—15 kilometers per hour. That means you need to be able to run 3.1 miles at a 6:25 min/mile pace. To make it easier to work out, you need to run each kilometer—0.6 miles—of the 5k race in 4 minutes.
I went from 33 minute to 20:30 5k in about 6 months of very dedicated training, and a decent natural body running composition (6'1 weighing in @ 145lbs). However, getting from 23:00 to 20:30 was equally hard, if not harder, than going from 33:00 to 23:00.
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how hard is it to run 5k in 20 minutes