Act as a certified running coach with 10+ years of experience training athletes for marathons. Provide a detailed 16-week marathon training plan tailored for [BEGINNER/INTERMEDIATE/ADVANCED] runners aiming to complete a marathon in [TARGET TIME, e.g., 'under 4 hours']. Include weekly mileage breakdowns, key workouts (e.g., long runs, speed sessions, recovery days), and tips for [NUTRITION/INJURY PREVENTION/RACE DAY STRATEGY]. Explain how to adjust the plan for [HILLY TERRAIN/HOT WEATHER/SPECIFIC FITNESS LEVELS] and address common challenges like [MOTIVATION/WALL HITTING/TIME MANAGEMENT]. Use clear, actionable advice.