I've finished mine up, and in hindsight, the writing was the hardest part-- if you're really paying attention to what you're saying, not just writing to get it done, your defense should go smoothly. The defense is nerve-wracking, of course, but it was much easier and less intense than I imagined. I was very very nervous going in, because I did a mixed methods approach, so I was planning on getting hammered in my data collection/analysis... I pictured my committee scoffing at my findings or tearing my research apart liberally-- but they were very professional and polite, and they asked really targeted questions. I did pick my own committee, though, which may not be the case at some schools.
Worst case scenario, if I hadn't have passed, they'd let me defend once more before telling me to retake the thesis hours. I don't know if your institution is the same, but at least theres a comfort to knowing that one mishap isn't the last straw at some schools.