I felt like this also! My knee started hurting at mile 2, at mile 25 I could no longer "run". I could walk and knew I would finish but I was going slow. The very worse part was I was running with a group of 5 (we were running for a charity) and I knew I was holding everyone back. I could not get out of my depression for days and was really surprised by my emotional reaction of sadness and frustration instead of joy for my accomplishment. I decided I'll stick to 1/2's for now.