Most experts agree that 20 to 30 percent of your weekly mileage should be devoted to the long run, depending on your overall mileage. Unless you're an ultra-marathoner, I recommend keeping your long runs to 3 hours or less. Any training benefit beyond 3 hours is just not worth the added fatigue and risk of injury. The new RDA for iron is 18 mg per day. If it feels too hard, it probably is.
Tempo runs and interval workouts are for building lactate threshold.
Long runs are not. They should be carried out at a comfortable, conversational pace. Follow your long run with a recovery day of very easy running, cross-training, or rest. Start your long run well hydrated and consume sports drinks and gels en route. This is excellent practice for the marathon and will keep you from getting overly fatigued.