To win basically anything in life you generally need to catch a break or two. To win the lottery you need to get extremely lucky. To win a marathon you just have to be the fastest person out of who shows up to race. If it's your local town marathon then you probably stand a good chance to win if you're skilled. If it's the Boston Marathon then you probably need to catch several breaks to win.
That is the idea of catching a break, it is something that is out of your control. Maybe the fastest runner in the world misses his flight to Boston because a storm knocks his power out.