After taking time off (a year) from racing and serious training, I lost my speed big time. Last month, I started reading Fast After 50 by Joe Friel. The biggest thing I took away so far reading the book is pushing your body with high performance. Doing long workouts is always at a slower pace but doing short high intensity can improve your speed.  Today, I set a goal of a time for my 5K which I beat by 8 seconds. And the best part I won my age group. Though I am training for an endurance event in September and Half Ironman, I will continue to work on my high performance with high intensity workouts. But not workouts can be high performance, you always need recovery/active rest to avoid injuries.