Should I cut back from 5 miles to train for a 5K?

As long as you feel fine during and after a 5-miler, keep on doing them. Your body is used to 5 miles, so it should think 3.1 miles is nothing. If I understand what you've said, you're running twice a week; is this right? If you decide to run more than twice a week, try something like this. Let 5 miles be your long run once a week. Let 3 miles be your medium run once a week. Let any remaining runs be 1.5-2 miles once a week or more. This will keep your body conditioned for the 5-mile distance and will give you extra rest to help your body adjust to running more than twice a week. An important thing in running is to run heavy/light. Sports docs say we need at least 48 hours after a run of heavy stress. The rest runs of 1.5-2 miles will give you runs of light stress. Space them (or days of no running) before and after your heavy runs of 3 and 5 miles. Heavy/light means you have a light-stress day after each day of heavy-stress.