Okay, here's what works for me. This probably would NOT work for a person beginning my program, BUT it works for me and where my training is at:

I know that I can run 12km at an average heart-rate of 165 without any problem. When I am finished, there will be plenty glycogen left in my body. I want to run my next big event faster. I use my road and mountain cycling to build more leg muscle endurance (and weight-training for muscle strength), but my cardiorespiratory fitness has to improve also.

I run with an I-pod mini and the different songs provide me with "intervals". The first song is the warm-up. Song two is High Intensity. Song three is stride work. Song four is back to High Intensity. Etc, etc.

I tend to run fast and sprint during the HI Intervals. I tend to do the long-endurance-cycling at a slower heart-rate to make sure the glycogen will last.

During my bigger events (which last 3-5 hours and I am giving it all I've got), I try to figure out just how much output I can achieve without burning out. I never run for more than an hour (max 12km) during an event. I am participating in a 24hour mountain biking event this year, but my total time of exertion should not exceed 5 or 6 hours (I'll get a few hours of rest while my teammates ride, several times during the event). I am planning to compete in a couple multi-day events for next year.

This is what works for me.

J.