I'm training for my first marathon. I'm up to an 18-miler and log about 35 to 38 miles per week. All of a sudden, I have felt so physically tired when I run, I can hardly make it through. I'm paying attention to my nutrition, and I'm taking GU on long runs. Debbie Marathon training does take a lot out of you, but it shouldn't wipe you out completely.
Women use and lose a great deal of iron through sweat and just being a woman.
If you drink caffeine, you might cut back and don't take iron with it, because caffeine inhibits iron absorption. This might not be your problem at all, but it's a place to start. Also, when I'm dead tired and I have a workout planned, I wait a day usually. You can't drive a race car on empty. I run 30 miles a week at a comfortable speed.