runningtipstoday:

I have been asked many times this fall and winter “WHY do I need to run 60-90 seconds slower than my race pace for the long training runs on Saturday or Sunday?”
The answer is that we are training your body to support you for 26.2 miles and we need it to adapt to provide you with the right strength and energy through the entire event. Speed work is where the body, through muscle memory, learns to run at race pace. But before you can be successful at speed work, you must have a solid foundation of base miles.