Why is my running endurance getting worse over time?

More Info: If you are fifty and asking this question, you should get the book "Running Past Fifty" by Richard Benyo. Not sure if it is still out there, as it was published in 1998 by Human Kinetics. Is it your speed or time or distance. Only very few keep their speed past 50. Distance and time endurance take longer to achieve and maintain. Rest is critical, recovery takes much longer.(63,Level I USAT coach)