If you are exercising harder then what your aerobic system can support, then your anaerobic system begins to help support the exercise. Improving the fitness of your anaerobic system requires intensity. A great deal of intensity. In the case of running, it means repeated efforts so hard that you're gasping for air and think you're going to die. What we generally do wrong is that we don't run hard enough.
Anaerobic fitness is gained fairly quickly and goes away fairly quickly.
Short sprints . Too much risk of injury and not long enough to do the anaerobic system any good. Usually they are the sign of a PT Leader that is not motivated enough to take his unit on a real run. Putting aerobic and anaerobic together . A good solution is to run long and slow 4-6 mi. twice per week.