i pressume you are a "new" runner as in you haven't start running for a long time and just started running lately

it would be your body is trying to adapt to your new exercise regime, so please be patient and you need to rest in between

in order to train for below 20min for 5KM, you need to 3:00 to 3:59 pace , you may follow this guide: https://www.runnersworld.com/advanced/a2082...-per-kilometer/

all the best to you