Whenever I have a big event coming up (mostly endurance events, not races) part of my training involves intervals. I've never taken a scientific approach to it and I don't have a heart monitor or even a speedometer. I will typically pick a tree or telephone pole far ahead and sprint at maximum speed until I pass it, followed by a recovery until the next telephone pole. Rinse, repeat. I've found that doing intervals on a regular basis increases my speed, which means decreasing my time on an endurance event.