It's a bit of a trope, by now, but, nothing new on race day! No new shoes, socks, shorts, shirts, foods, etc. Do the same as you do on training day.

Pee as late as you can before the start. A nervous wee before the race can save you queuing up mid-race.

Ignore the pace of crowd. Make your first km your slowest. It will set you up for a strong finish.

Look around. Enjoy the sights. Say hi to your fellow runners: there are some amazing people out there and a good conversation can help you go the distance.