Good thread, I ride Mt bike and Road 4-6 hrs a week,
my thoughts are:
Hydration is important, especially in SOCAL (where I ride) try a Camelback makes it very easy to take small sips often.
Spin faster rather than pushing hard all the time.
One rule of thumb for position is to sit on bike w/ pedals at 3:00 & 6:00 and have a friend hang a weight on a string from the bump just under your kneecap of front foot. The weight should be in the center of the pedal, adjust seat to for allow this.
Finding others into this makes it even more fun and they act as support group; technical, emotional & competitive.
Once you feel like you are getting in very good shape and want more, start heart rate montioring. you can be tested to detirmine your max heart rate or use less accurate calculators on web.
http://www.healthchecksystems.com/heart.asp
The basic idea is that your fitness level will increase faster when you spend allot of time training at 60% to 80 % of max heart rate.
This did wonders for me. Mountain biking can be more about stamina and pushing limits which is fine but building a solid base will help overall enjoyment (you know FUN) and fittness.
Also try http:/thecyclotherapist.com/links.html for allot of places to go for ideas help etc...
This sport is a blast, don't get hung up on equipment, buy what you can afford and enjoy the heck out of it.
Bicycling magizine is a good all around mag, lots of self help stuff.
ciao