Poor excuse for a run

Today's run was by far the worst run of my training to date. I was meant to run 14 miles. I slowly jogged/struggled through the first 7 and 3/4 at which point I decided I just could not go any further and so I walked. Once I hit 8 miles I found a bench and literally sat down and read some email. I was about half a mile from my apartment so I toyed with cutting my losses and just going home, but decided I really should finish the last 6 miles. So I got back up and was on my way. The last 6 were just as painful as the first 8, perhaps even more so. Through some combination of horrendously slow jogging, walking, and sitting on benches I managed to cover all 14 miles. It was not pretty. And I'm bummed, to say the least. The past few weeks have been awesome. This week has been a constant struggle. I need to figure out what's going on here, especially since I'm in the home stretch. Just 4 weeks to go until the marathon. Hopefully I can get this sorted out sometime soon...