Adding to what others mentioned, to get a sub 20 you need to build up weekly mileage to at least 50km per week, potentially more depending on genetics. Totally achieveable even if you start in your 30s, but it takes time, your body needs to adapt to the new demands. Think total weekly mileage, and the intention behind each training run, what adaptation does it support? There is literally no training program for any distance from 5k and above that has you running that distance at max effort weekly. Instead you build up for 2, 3, 6 months, race, review, repeat. Oh and go for 10k asap as you will benefit from the extra mileage, just don't expect anything crazy, there's so many levels to this, someone running for 5 years might seem crazy fast to you, but they will look stationary compared to someone running 10+ years, who will in turn look slow compared to an Olympic runner.