Go to a track and run 5x800m with a 200m easy jog rest. Whatever your average pace for those intervals will be a decent predictor of your race pace. The closer they are to each other, the better the prediction. Start the first one at a 20-20:30 5k pace (around 6:30 miles, 3:30 800’s. 1:45 laps) and try to hit that pace the same each interval. The first should feel easy and the last should feel bad.
Not being a dick, but the math is off a bit i.e. 3:30 800m vs 6:30 mile.
Having said that an easy and quick way to get a close guess to 5km pacing.
If you are close to 20 min the 48 sec per 200 is a 3:12 per 800 or 20:00min flat for 5km. For people who can’t pace I just say set your repeat timer on 48 sec and hit those every 200m.
Then take 2-3X 48 sec rest or easy jog/walk on the 200m
Maurice