Before adding intervals it is imperative that you have a solid base of aerobic conditioning. Failure to follow these guidelines can result in premature overtraining of the sympathetic nervous system otherwise known as Basedowic overtraining. This type of overtraining is characterized by sleep disturbances, reduced appetite, accelerated resting heart rate, increased resting blood pressure, and/or a drop in systolic blood pressure upon standing (i.e. orthostatic hypotension). If you're like most motocross athletes I will assume your aerobic conditioning to be more than well-developed and get to the question at hand.
Interval training programs are limitless in their design possibilities as they are composed of an infinite combination of work time, rest time, intensity, repetitions, etc. If properly structured, intervals can certainly be adapted for longer....

.....duration events such as Hare Scrambles. However, the "ideal" characteristics of any program will vary according to demands of the given sport. For example, short-duration, high-intensity activities such as Supercross require the ability to tolerate extreme cardiovascular overload and lactic acid accumulation. Therefore, the work effort must be intense (greater than 100% VO2, at or near maximal HR) and the rest interval brief.
Long-duration, "moderate"-intensity activities such as Hare Scrambles are much less strenuous but require that submaximal effort be sustained over a longer period. Thus, an interval training program designed for this purpose would have longer, less-intense work periods (relative to Supercross anyway) with medium-to-short duration rest intervals.
What is crucial for both programs, or any interval program for that matter, is that they utilize rest periods that do not allow for complete recovery. The physiological changes must accumulate during a workout or the energy systems (both aerobic and anaerobic) will not be overloaded.
Remember, the purpose of any auxiliary training (weight lifting, interval training, mountain biking, etc.) is to raise the capacity of individual systems (i.e. muscular, cardiovascular) to an extent not developed by the sport itself. If whatever you choose does not accomplish this, you'd be better off just riding.

Reprinted with permission from Steven Bubel.   For more great Q&A's, visit  Steven's website at