Aerobic Swimming Speeds - How Fast Should I Swim?

When lifting weights, you have a pretty good idea how the work should be done. Up, down, a certain number of repeats with a certain amount of weight, maybe repeated in several sets. It may be based on a percent of the maximum you can lift or on a history of what you lifted previously. In the swimming pool, how do swimmers know how fast to swim to get the desired training result? This depends on what result you want emphasized - anaerobic or aerobic metabolism.