hi there, i am training for Belfast marathon, done my 18 in what should have been 3 hours or just over, and only managed to do it in 4 hours, i have been steadily training, ok missed a few sessions this week, but the first 10-12 mile was fine, but the PAIN hit hard the last 5 or 6, is this due to under training or is it a unevitable thing that will happen when you reach that mileage??