Another 450 calories burned. If you go out tonight, get home early. Today's workout will take you 7 miles. This will be a significant point in the half marathon itself, because you will be past the halfway point. When you see the 7-mile marker, you can tell yourself you've got fewer miles to run than you've already done.
Psychology can play a big role in covering the distance.
Run comfortably today. If you need to walk in the middle of the workout, don't feel guilty. You may need to do some walking in the half marathon, so it's important that you become used to moving from a running to a walking mode and back to a running mode. It's best to walk on your schedule, not because fatigue forces you to walk. Swimming is a particularly useful activity, because you can use it to loosen your muscles.