Wow, Ben, there’s a lot going on here! I wish we had a specialist in non-traditional multisport race events on staff here to supply you with truly expect answers to your questions. Alas, we do not. That being said, some principles apply to all endurance sports disciplines, and so, speaking as a person who has never kayaked, I point you toward these.
The body runs on time, not distance. If you want to be well-prepared for an endurance event of sketchy distance, train to be prepared to perform well at the maximal likely durations of the various legs and of the race as a whole.
Whether it is helpful to train at distances/durations exceeding the race distance/duration depends on the length of the race, but unless it’s a very long race, it probably is. You don’t want to do these over-distance sessions at race-intensity, though. Separate your endurance-building sessions from your higher-intensity sessions as you would in any other endurance specialization.
I wish we could be more helpful at this point. We’ll get there! And in the meantime I invite others in the 80/20 community to offer Ben additional guidance.