Before you start training for strength you need to build a base of fitness. Part of this is to do the exact opposite of what a lot of people on this thread are saying and spin like mad. The BEST way to train for endurance is to get yourself a heart rate monitor and train in the appropriate zones. Then put in lots and lots of miles and time in at the appropriate zones. So rather than riding a low gear because it's easier, you ride a lower gear to up your cadence but ride it hard enough to keep your heart rate up.
Once you build yourself a good cardiovascular base, then you can start doing other stuff to try to make your training more specific - the types of things other guys have been talking about.
Check the training section of Bikeforums.net:
http://www.bikeforums.net/forumdisplay.php?f=148
The VeloNews training tips:
http://www.velonews.com/train/
This is a good article:
http://home.hia.no/~stephens/mtbplan.htm
Bicycling.com's training section:
http://www.bicycling.com/article/lis...ategory_id=363
There is a definite science to training. It's your choice how far you want to dive into the science. You can go out and drop $1000 on a computer w/ power meter, heart rate monitor, altimeter all integrated together that communicates with your computer so you can collect and graph data, then sent that data off to your personal coach at Carmichael Training Systems............or you can read all the stuff and just use it as a general guide of stuff to keep in mind while riding for fun. Or anywhere in between.
Even if you do it informally, a HRM is very helpful. I don't use a distance/speed computer at all but rather a mid-range programmable HRM. The graphing heart monitors or even just the ones with programmable zones that you can play back on the monitor are pretty darn cool. But just a basic one that simply displays your current heartrate is well worth it. Like this basic Sigma:
http://www.performancebike.com/shop/...egory_ID=4115#