Why run a marathon?

I'm having real doubts as to why I keep paying money to run marathons. Each race brings with it weeks of training, running miles each week, etc. Before every race I seem to hit the wall before I even start running. Crazy. I'm sicker than I've been in a while with a nightmare cold, cough, and congestion. I've worked so hard putting in long miles each week and now I feel mostly dead. Not from the training, but from a virus. UGH! I should be grateful I can run at all, but I want to run well at these marathons. I want to qualify for Boston. Why is it so hard!!!!