I find myself practicing many hours at a time. so far i dont have any wrist problems, but should i worry about tendonitus? i know that if you have tendonitus, then you cant play at all and im concerned. are there any warm ups or exercises i should do to prevent me from having this? thanks!!

You obviously should warm up and down. It isn't true that if you have tendonitus you can't play at all. I have tendonitus, have had surgery on my wrist twice, and I still play 4 to 6 hours a day. As far as exercises go your warm up should take you through the range of motion you are using slowly. I.e. If you are using Stevens you should go through block chords (Or anything with that motion) and also an independant motion.
Talk to your doctor about your hand concerns. She may be able to help prevent problems before they start.
Jen