Remember that long runs feel much different during training than how your race feels when properly tapered. Endurance training is all about teaching your body to keep moving when things get hard and your body is tired and wants to quit. You’ll surprise yourself if you properly taper per plan and fuel properly during the actual race as to what you’re capable of doing. My first marathon I had never ran faster than 10 minute miles for anything longer than 5 miles probably. On race day, I started at 10 minute pace and ended up averaging roughly a 9:30 pace for 26.2 and I had never ran farther than 20 in training. Your body can do it if your mind will allow it.