Over the course of a race, it takes increasing physical and mental effort to maintain an even pace. You can meet this challenge by keeping relaxed and in control, and by increasing your concentration as the miles go by. Keep thinking about what you're doing. Keep mentally checking your running form, your breathing, and your leg speed. One of the most difficult parts of racing a 10K comes between miles 3 and 5, where the exuberance of the start is gone and the finish seems so far away.
Mental tricks can help here.
Think of the race as segments, and concentrate on one part at a time. When you're at the 3 mile mark, think only about running to the 4 mile mark. When you reach the 4 mile point, take a few deep breaths, be happy that you made it that far, and focus on making it to the 5 mile mark. This approach is easier than thinking about the whole distance before you. The last mile, like the first, should take care of itself.