If you’ve been feeling unaccountably worn down for at least a few weeks, you could be suffering from iron deficiency. When your iron is low, you’ll lose enthusiasm for running and most of your runs will feel like chores. Faster running will be especially difficult; your times for hard workouts and races will get much worse.
It’s common when feeling like that to have your blood checked. Trouble is, the measures usually looked at, such as hemoglobin level, can be within the acceptable range, and then you’ll start thinking you’re a head case or have something much worse wrong with you. Ask to have your serum ferritin level checked; it’s usually not included on a standard blood test. Serum ferritin is a measure of your body’s iron stores, sort of like the nest egg you’re not supposed to touch unless things get dire. It can show problems with iron storage and absorption even if more standard measures of anemia are fine.
If your serum ferritin level is below 30, it’s probably affecting your performance. Increase the amount of iron-rich foods in your diet and you should feel better within a month. It’s a good idea to have your serum ferritin level measured when you’re feeling great in your running. That will give you a baseline to compare to for future readings.