You're not cheating yourself if you ride a shorter distance than Group 1. If we all rode the same distance, Group 6 would end up training at the lowest relative intensity. For this ride, we're training by time, not distance.

The causes of muscular fatigue are not fully understood, but a major component is biochemical. One goal of training is to deplete biochemical substrates to stimulate adaptations that increase the size of stores and the recovery (replenishment) rate.

Fibers are recruited by the size principle. The endurance of fibers varies on a continuum of seconds to hours. At any given intensity, the body preferentially recruits fibers with the most endurance. So, intensity targets the muscle fibers that will be trained and sufficient time at that intensity triggers adaptations.

For any intensity, we can only train a subset of our muscle fibers. Either the intensity is too low such that a fiber isn't being sufficiently recruited or the intensity is too high such that sufficient duration can't be maintained to train a fiber. To train all our muscle fibers, we must perform a continuum of different types of intensity/duration training.

Another way to think of the power/heart rate zones is in terms of time trial pace (10TT = 10 min time trial pace or all out, steady 10 minute effort).

Zone 5: 10TT-30TT
Zone 4: 30TT-90TT
Zone 3: 90TT-180TT

From this perspective, time = intensity. If we all train to the same duration, we can all train at the same relative intensity.