Leg and Oblique Cramps

so, I was running for a sports day that is coming up in which i am running 800m. Yesterday, was the practice run, as I have never ever run properly before in my life and am overweight, I was quite surprised when I came in second place! I honestly still can't believe it but anyways,

after that run, and a swim meet, my legs and obliques are tired, sore and drowning in cramps. the real sports day is coming up soon, so I was wondering if anyone could link a workout that I could do to help diminish the cramps?

xxx

AwesomeM