For a 5k you do not need EXTRA carbs before you run, that is a straight answer.
Carbs take about 30 minutes or so to work their way into the relevant bits of your body. Having said that I often have a 500ml lucozade sport in lieu of a banana-breakfast before a 5k and it doesn’t do me any harm.
Just eat enough to avoid feeling sick if you suffer from that. Don’t have a huge meal the night before, you’ll just have to carry the extra weight. By experimentation, work out what you need to eat &/or drink as a minimum intake well before the race day itself.
Bit of protein, bit of carbs for your muscles and a rehydrating drink when you’ve finished the race, sorted.
If you want more detail on this matter then look at the article below by Joseph Agu.
Also of interest if yo are really bored is the legal ruling on Lucozade (which incidentally I think is wrong!)