Sometimes our desire to improve gets ahead of our physical capabilities. Good practice will regularly push the limits of our skills, but we must also take care of our physical health. A healthy body can only be maintained with a healthy mind.
Practice Monster has never heard of tendonitis, and he could care less about things like pinched nerves and repetitive motion injury. Getting carried away can be a deep and meaningful experience, but injury is a steep price to pay.
It is always a good idea to break up long practice sessions with little breaks. Try some type of physical exercise, like a sport, yoga, Alexander technique, or whatever you find interesting. Most importantly, listen to your body. Practicing in pain is a very bad idea, highly habit forming, and a sure way to summon the monster.