Meaning: This rule suggests that the perceived effort during running should generally be moderate, especially for regular training.
Mechanics: If running consistently feels exhausting, it could indicate overtraining, improper pacing, insufficient conditioning, or that the intensity is too high for the intended purpose (e.g., endurance building). Moderate-intensity running allows for recovery, improves aerobic capacity effectively, and is more sustainable long-term. High-intensity or 'exhausting' runs have their place (e.g., interval training), but they should be programmed carefully and not represent the entirety of one's running routine.