Better technique also makes it easier to endure 5k. 20:00 for a 5k is a good starting target for a reasonably fit person; that’s 2:00/500m split time. I’d expect a games athlete to be in the 18s and seriously quick rowers are faster (C2 lists 14:54 for men’s heavyweight). Your height is a disadvantage.

Good runners are efficient (meaning use less oxygen for a given running speed) and have the ability to use more oxygen per minute than untrained people (up to 5 litres/minute compared to untrained people who max out under 2 litres/minute). Running as fast as you can for 5K does not specifically target aerobic system, hence the need for easier
Women, on average, tend to run a 5k with a time of around 35-45 minutes. Consider the speed at which you can run a single mile. If you can run a mile in about 10 minutes, then a 30-minute 5k race goal might suit you. If you can commit to some serious training, you might be able to increase your running pace and surprise yourself. A Good 5k Time

*I* (47M, I run for fun, stress relief and health) run 70 miles per week and 35 minutes for 5k is a good day for me, it means I am doing better than 12 minutes per mile. My best 5K was 28 or so and it didn't kill me, it was kinda fun. The good news is that if you can do this, you can get better at it. But 35 minutes isn't horrible, it's just a

This 5K training plan is designed for runners who are looking to finish at a time under 24 minutes. It's designed for runners who are finishing 5K's under 27-25 minutes already but need a little extra structure and guidance to get to the next level. This is an 8-week training plan that offers interval training days, hill repeats, fartleks
158 is really low for 6:28/mi pace. I'm 6'3" with a 34" inseam and when I started running my cadence at my comfortable easy run pace was high-150s, too, and went up to about 165 at my fastest runs. A couple years later, my easy run cadence is about 163-165 and goes up steadily as pace increases. I also achieved a personal goal of a 20:00 5k n6pZ. 311 275 26 218 224 258 78 132 179

is a 21 minute 5k good