Hi Klauss,
I recently started school, (around June/July), and this was a concern for me at the time. But being a procrastinator, I didn't do anything and honestly-it was fine. We only practice for an hour or two each day of classes, so endurance wasn't really needed.
Now that I have been in school for a few months, however, I have been learning about the importance of all the different aspects of health. I have gotten more serious about getting my body back in shape. Particularly so that I can stand the stresses of doing massage full time in the near future.
In my opinion, yoga is great. I love it and recommend it to others every chance I get. I think that yoga and Tai Chi are especially good for MTs because the stances in your yoga practice are similar to the body mechanics you must practice when you give a massage. It also helps you to listen to your body more carefully, and that, in my opinion, will help when you have to relate to other bodies.
Being a female with thin arms, I have also found it to be very helpful to build strength in my forearms by using dumbells.
Bottom line is that best way to be prepared is just to find what works for you. Health seems to come from a combo of healthy eating habits and regular exercise. If you don't figure it out before school, I'm sure that you'll have plenty of resources once you get there!
Hope that helps