How do I build the fitness a combined driving horse needs for the marathon?

Building marathon fitness requires a conditioning program that develops both the aerobic cardiovascular base needed for sustained road and track work and the anaerobic capacity for the intense efforts of hazard negotiation. These two fitness components are developed through different types of work and require different training approaches. Aerobic base conditioning is built through sustained moderate-intensity work — long drives at road and track pace over progressively increasing distances as the competition season approaches. A horse that can maintain its working trot pace for thirty to forty-five minutes without significant quality degradation has a cardiovascular base that makes the road and track sections manageable. This conditioning is built over weeks and months of progressive work, not in the final preparation period before a competition. Interval training — repeated short, intense efforts at a pace above the road and track pace, separated by recovery periods — develops the anaerobic capacity that hazard negotiation requires. Each hazard section demands a burst of intense, athletic effort that the horse's aerobic system cannot fully support, and the anaerobic capacity to produce that burst and recover quickly for the next hazard is a specific fitness component that sustained moderate-intensity conditioning does not develop adequately. The timing of the conditioning program relative to the competition season is as important as the content of the conditioning work. Building fitness progressively over months and then tapering the work in the final two weeks before competition allows the horse to arrive fresh and fit rather than either underprepared or fatigued from excessive work in the final preparation period.