Thad Jones and Cecil Bridgewater waiting for Bullet Train in Japan abt. 1974
How do I practice for long periods of time without injuring myself?
First of all you don't set out to practice 8 hours a day until you have worked your way up to 8 hours a day. An athlete doesn't go into the weight room expecting to lift 500lbs. on his/her first day. You have to work your way up to heavier weights - why would you think it was any different when trying to accomplish the same fete's on an instrument? One of the dangers in trying to put in a great deal of time without the proper preparation is that you start doing things the wrong way and you keep re-enforcing doing the things the wrong way, i.e. you get better and better and doing it wrong.
You have to be willing to correct yourself when you sense that you are developing a bad habit. Unless you have a teacher/observer there in the room with you at all times - you have to be the one who recognizes that something is not right in your approach. Essentially, we are all self-taught, because our job to ourseleves is to self-correct!
THE DIFFERENCE BETWEEN
A GOOD MUSICIAN
AND A GREAT MUSICIAN IS ATTENTION TO DETAIL!
Spend a proper amount of time warming up before you try to do the musical gymnastics that you hear others doing.
Don't try to do more than you are capable of at this moment, but realize that you are striving to get to a certain place with your musical development.
Keep records of your progress: a journal of your daily practice routines - warm up exercises, scale exercises, tonguing exercises, slur exercises, etc. your goals (where you intend to end up at the end of a session/week/month/year/5years/10years, a career etc.) what tunes you know and what ones you need more work on what new ideas you have come across - musical and business - write them down so that you can go back to them continue to listen to the great musicians and learn from them - don't limit yourself to only a few, but listen to everybody even those that you don't particularly like and you'll find out some things from them that can be valuable
As you build your musical vocabulary keep working on the mechanics of performing on your instrument: proper breathing, tone production, finger control, range, volume, sight reading, section playing, interpretation of differing musical styles
Unfortunately and fortunately the learning never ends and the working on all of the things that will make you a better musician never ends. It's an on-going process that will follow you all the days of your life!
The more you know the more you find that you don't know and then you find that there is a whole world of things that you don't know - so keep working on being a better person and the musicianship will follow!