I didn't get a period for about a year between college and grad school. Over the course of a year I lost 60 pounds and was running regularly. While I was healthy, I wish I'd been more cognizant of the effect of missing periods on bone density, etc. Your body apparently can't absorb calcium without the hormones (estrogen I believe) present in a regular menstrual cycle.

so... You may be perfectly healthy amenorrhaeic, but see your doctor to be sure they don't want you supplementing with some hormone to make up for the loss.

Interesting that your base training is 100 miles per week higher than your race period (so to speak).