Vino, I wouldn't be surprised if you could knock it out immediately just based on what you've done these past few months - you certainly have a more than decent aerobic base. I know you just did a 21min 5k, but really that was on the heels of a lot of miles in the days immediately preceding. So, it's probably right there without too much more to do, but to really knock it out, some sharpening is in order (since like 95% of your runs are "easy" at this point).

I would keep doing what you're doing, but work in 2 harder days - either a tempo run (a few harder miles) or some intervals (either structured like 6x800 or 'informal' like surging for 2 or 3 minutes a few times inside a longer run. And since you'll be dialing up the intensity, you could consider backing off a little on the longer runs (but you don't necessarily have to - would feel that one out).

I really don't think it'll be a long journey to sub 20 for you.

Hi Vino I live down in Papamoa 

When Im 5km training I do 6 x 1000m intervals once a week at 5km race pace with 2 min jogging rests. They are hard but replicate the toughness of a 5km race and give you an accurate idea of your fitness. I do the thursday night 5km race down here but only every couple of months to give myself time to improve.

I think 50 mpw is a good allround mileage for 5km racing.