If you ever were taught how to play an instrument, did you learn how to play simple pieces before learning more complex pieces? Did your music teacher constantly change the piece and complexity of the music that you practiced, or did he or she have you practice the same piece of music until you mastered it?
After repeated or prolonged presentation of a specific stimulus, you become habituated to it, and your body decreases its response to that stimulus. Confusion, on the other hand, keeps your body guessing by constantly varying the stimuli. While “confusing” your body can be useful to avoid plateaus in fitness and performance, variation to cause confusion must be balanced with mastery of the skill. On one hand, you must vary your training often enough to adapt and improve fitness, while, on the other hand, you must repeat the same training a number of times to master the volume and intensity (or to master the skill of a specific type of workout) so you can progress with your training, having each workload build on what came before.
Habituation, a learning process that leads to mastery of a skill or workload, is a more effective training method than confusion, as long as the same stimulus is not repeated for too long that the physiological response begins to decrease. Give yourself enough time to absorb and adapt to the training before changing it. Forty miles per week should become a normal experience for your body before increasing to 50 miles per week. Change the stimulus just as habituation occurs so that you continue to increase your response. Most runners would benefit from changing the training stimulus every few weeks.