Do almost all of your fast running at your current race paces, not your goal race paces. That will ensure that you’re running within yourself and working at the proper intensity, rather than straining to run times that might have no relation to your fitness. After all, if it really worked to pick an arbitrary time and start training at that race pace and inevitably run that race time, well, then excuse me while I go start doing all my hard workouts at sub-4:00 mile pace.
First, do the training at your current capabilities. That will advance your fitness. That will lead to faster race times. Adjust your workout times only after you’ve run the faster races that justify them.