D,
Great question. The purpose of training is to improve your muscular, cardio vascular and neurological systems. Your brain must be trained to resist fatigue, and that level of brain training only occurs in workouts of around 2+ hours.
Given that your Sprint will take ~1.25 hours, regular 1.5 hour sessions is not extraordinary (the max run duration in the Sprint plan is 1.5 hours). The 2-hour bike rides are really the sweet-spot for neurological benefits.
We even have 1.5 hours runs in our 5K plans! Professional baseball pitchers, who run 0 miles in a game, will run 45 minutes a day for this same brain benefit. The bottom line is that training your body to resist fatigue can only occur at these minimum durations.
David