Another set of training musts are intervals. These are akin to rigid fartleks -- you increase your speed for a set period of time or distance and then recover for a set interval before ramping it up again. Intervals have a dramatic impact on your conditioning; in one study, it was found that just two weeks of interval training helped six out of eight study participants double their endurance [source: Jaret].
While there don't appear to be any hard-and-fast rules about how long your intervals should be, a general guide is that during the intensity phases, you should be working at about 80 to 85 percent of your target heart rate and be too out-of-breath to hold a conversation. After those bursts, your recovery phases should let you catch your breath, but your heart rate shouldn't drop back to its normal level.
Still not convinced about intervals? How about this as a motivator: Intervals burn significantly more fat than running at a steady pace.