I am a PhD candidate in Church History/Theology. I am in my fourth year of study. I have 69,000 words out of 80,000 written on my thesis. I am attending a British university where the PhD is 100% research, no classes.
Many of my masters-graduated friends ask me if they should pursue the terminal degree. This is my answer to all.
If you can do the following: you can make it.
1. Study. I mean study for hours a day.
2. Limit your social obligations. Here it is New Year's Eve and I am in my room with the door locked. I am not partying. You will have to give up many, many parties, events, that you once attended.
3. Rewrite. I remember vaguely the days of roughdraft-edit-final draft. Any paper or chapter that is a finished project has gone through at least eight drafts, usually more.
4. Take criticism. If your advisor/supervisor is doing their job, they will rip your work to pieces in the beginning. You have to be able to take it.
5. Be disciplined. When writing a thesis/dissertation, you have to set your own deadlines. No one will make you sit your butt in the chair and type except you.
6. Be humble. Education is a long lesson in humility. If you think you know everything you need to know and just need the letters at the end of your name, forget it. You won't make it past the first semester. The more you know, the more you will realize what you do not know.