I worked upwards of 80 (sometimes 90) hours a week for most of last year. At the end, I was exhausted and hibernated for a month.
Keep your schedule as regimented as possible. It's easy for your time to slip by and to have things take you unaware. It's also easy to want to sleep in some of the days, skip work, etc. Don't do that unless you have to. Set yourself a regular bedtime, stick to it.
On the weekends or when you have time off, it's important to keep some social interactions in your life. That being said, sleep and rest should be prioritized so if you're not keen on meeting up with friends, cancel and take care of yourself first.
For me, I don't think I could have gotten through it without running in the morning. I got up two to three hours early and just went running for a long, long time. It cuts back on sleep, but I found the stress relief and the ensuing runners high was more than enough to make up for it. It helps you center yourself. Obviously you don't need to run that much, or run at all, but physical exercise is going to be critical.
Keep your eyes on whatever your future goal is and be sure to treat yourself. It's easy to get overwhelmed by the sheer amount of work you'll be doing, but I presume you have a reason for doing so. Keep that in mind, and splurge (when you can afford it).
Be sure to stay on good terms with your coworkers and be upfront about it if you think you'll be tired, irritable, etc. Spending that much time at work with people you don't like is a death sentence.