How can I get through the 1,500 meters?

I am not very athletic at all and tomorrow is my school's sport's day. So far, I've ran the 800m every year so far but now, I've found out I have to run the 1,500m. I can barely get through the 800 m and I have to walk part of it sometimes but at least I can handle it. I'm terribly frightened and I'll take any advice you can give me on this race.