When you train with distance (slow to medium pace) it is aerobic, i.e. in the presence of oxygen.

When you do intervals (sprints) it is anaerobic.

I would say that BFL aerobic training moves from the former to the latter as the intensity increases.

Same with jogging/running vs. sprinting.

Now as for the catabolic thing, I understand that the longer the time period involved the greater the chance that it becomes catabolic. As you deplete glycogen the body moves to break down proteins to get the energy it needs. Fat can also be broken down, but it is not a simple case of moving exclusively from one to the other. When you are low on glycogen often some protein (via muscle) gets broken down in addition to fat. According to Atkins there are ways to eliminate this but according to my GP and swim coach you are still going to breakdown muscle with extended exercise if you don't take enough carbs.