Expend as little mental energy as possible in the first 15 miles of a marathon. That doesn’t mean to disregard your pace or be nonchalant about taking drinks or not monitoring how you’re feeling. It means that if you’re too engaged mentally, like responding to every move made by runners around you, then you’re too caught up in it too early. You’re not going to be able to hold that level of exertion, both physical and mental, until the end. Anyone who has crashed over the last miles of a marathon, me included, will tell you it’s an experience worth avoiding.
If you’re properly trained, the pace in the first half of a marathon should take care of itself; as much as possible, put your body and mind on autopilot. If you have the common experience of the pace feeling so easy that you go faster than goal pace without trying, restrain yourself. Running at that slightly faster pace is burning extra fuel you’ll need every ounce of after the 20-mile mark. Save the pace pushing and heroics for the last 10K.