Sorry if I put this in the wrong thread, just joined!

For sports day (im female, aged 13) im running relay, 800m and a mile. I'm not bothered so much about the mile because its not on the actually sports day and neither the relay because its a group effort and only 100m. But im really worried about 800m because well its infront of the whole school and I will feel really really embarressed if i come last or dont come 1st 2nd 3rd, i would hate it if i get left behind abit. Its on next thrusday anyway, and iv only just started preparing for it, but running 2 mile today on roads and afew planks/sit ups, but not much. I am one of the most 'sportiest' ones, thats not a wors but you know what i mean. So basically what can I do to work up my stamina and get a faster speed, and any tips in general? Thankyou x