I racently ran a half marathon. When I finished the race, I was exhausted and pretty out of breath and a bit incoherent, but I was still able to walk in a straight line and didn't need any help staying on my feet. My legs were shot and the bottoms of my feet hurt and everything hurt. Is the fact that I didn't keel over at the finish line and that I was able to walk indication that I didn't give it a full effort?