Many people, especially many «athletes» are running long distances with a very
poor aerobic system, relying heavily on their anaerobic system to get them through their training.
However, your average american is almost guaranteed to have some level of carbohydrate intolerance (think: insulin resistance), which is a sign of a very
poor aerobic system.
Not exact matches
In other words, the distances you are running may (or may not) be slowly chipping away at your
aerobic fitness NOT because your
aerobic system is
poor, but because for X or Y reason your body is not able to recover fully before the next run (or the next week of running).
For example, if you're out for a long run, say 30 - 60 minutes, and your
aerobic system is very
poor, then you might be running at say 70 % anaerobic and 30 %
aerobic.