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I want to be challenged and feel like I've accomplished something when I'm done. So I pick the hardest stuff I can find and work the heck out of it. Mostly trumpet solo music that is all over the register and very fast. It's fun and quite a work out...I am frequently wiped out after an hour. I find most practice music simply boring (probably get bagged for that statement).

Tekknofieber,

in a way, I´m in a similar situation. From what you write, I figure you´re on your own as far as practising/learning tuba is concerned, so there are no studies assigned to you to prepare for a given date.
If that´s the case, your only shot is to teach yourself.
Record yourself and have musically well informed people listen to you. (Not necessarily tubists). Define your weaknesses.
Make up a set of goals on your own: Range ? Tone ? Air flow ? Phrasing ? Whatever ?
Try to find strategies to improve on your own, write these down and stick with them. If necessary, invent your own studies.

It´s a good idea to check with a pro tubist / tuba instructor at least on a semi-regular basis (as in: every second month).