49f-runner 5 mi 4-5 times a week. Pulse under 50, 50% of the time wornout?

Fuel and don't push. It's hard to say without more information about your workouts and your running history, but you could not be eating enough/right, be overtrained, burned out, iron deficient, or sick. Ease up a bit and make sure your nutrition is adequate. If it doesn't improve, or gets markedly worse your doctor can evaluate you.