boston memories

In my mind I'm ready...I'm prepared i just can't be scared. I've trained for months even years i couldn't quit if I dared. We are at the start and im think run smart. I can make it to the end eventhough im just blending in. The day is beautiful, the crowd is thick, but some tension in the air is very thick. i give my pace a gaint lift then BOOM! feels like i've been hit. What is this pain i see nothing around as i lay on the ground. Crowds are scattered, and im on the ground laid out like a stick. Bomb the lady that ran passed me said and out of consious i drifted. What just happened this day in boston?