Racing half marathons every month is the best way to get into shape, right? Not so fast. There's something to be said for racing often, but if your goal is to PR, it's best to choose a few key races throughout the year and build training around them. If you're set on, say, April and November marathons, make sure to plan out enough time for base building, peak training, tapering and, of course, post-race downtime. This is called periodization and the training peaks and valleys allow you to focus on the races that matter most.