I was the child that came last at cross country in school.  

Actually, to be fair to myself, I was third from last.  The child with asthma and the child that wasn't trying came behind me.
I missed the ball in rounders, my passes in hockey were pitiful, I wasn't strong enough to get on the trampoline, let alone brave enough to do a seat drop and I liked being Goal Defence in netball so I didn't have to do as much catching (or in my case, missing).
One year my sports day team captain put me down to run the 800 meters. 
What did I do?  I paid a girl in my class £4 to run it for me.