Optimal Intensity training ratios
Long slow distance is great for building up a strong aerobic base (circulatory and respiratory systems), if you have the time. However, unless you choose good rides it can be boring and few people would consider doing it on an indoor trainer.

For those who don't have that time, there's a century training plan in Chris Carmichael's Time-Crunched Cyclist and most people would consider a century long-distance.

However, if you're not so concerned about getting a great time, then the amazingly-simple way to train for longer rides is to go out and ride, each time slightly increasing the distance. A bit more detail on long-distance riding is given on YACF.