Threshold pace is the pace beyond which your blood begins to accumulate lactic acid at an accelerated rate. By running consistently at this "comfortably hard" pace, you can raise your lactate threshold without unduly risking injury.
Cruise intervals are a specific type of threshold-pace running in which you divide the workout into several segments of accelerated running (three minutes to 10 minutes worth of running at roughly 15 seconds over your 10K pace) separated by short recovery periods. As a result, the lactic acid level in the blood remains quite constant.
Cruise intervals should comprise about 8 percent of your total weekly mileage or three miles, whichever is greater. Marathoners in training usually run a greater number of cruise intervals than milers because their weekly mileage is usually greater. Their pace, however, is figured the same way.
For a middle-distance runner, a cruise-interval workout is a relatively easy training day in terms of intensity of pace.
But cruise intervals are perfect as a second quality workout in a week where you do shorter, harder repeats, such as 200s and 400s, more specific to actual race pace.
If you mix in some tempo running with your cruise intervals and more intense speedwork, you should cruise to a PR in no time.