The single most common marathon mistake is starting to train too late. This calculator tells you exactly when to begin, how long each phase lasts, what mileage to target each week, and what your longest training run should be — based on your race date and current experience level.

Marathon training is structured into four phases: base building, development, peak training, and taper. Beginners need 20 weeks to safely build from modest weekly mileage to the 28–32 km long runs required to finish comfortably. Intermediate runners with an existing base need 16 weeks; advanced runners with 70+ km weekly mileage can peak in 12 weeks. Compressing the schedule risks overuse injuries.

The 10% Rule — never increase weekly mileage by more than 10% week-on-week — underpins all three plans. The taper phase (reducing mileage in the final 2 weeks) is equally important: it allows your body to store glycogen, repair micro-damage, and arrive on race day fresh. Skipping the taper is one of the most common mistakes even experienced runners make.