One of the most effective cardiovascular workouts is one that involves interval training also known as overloading.  Interval training challenges both the aerobic and anaerobic systems enhancing optimal cardiovascular conditioning.  Performing interval cardiovascular training will bring substantial results to your body.

According to Paul Robbins, author of Personal Training on the Net:

Overloading is necessary if your goal is to bring positive physical changes to your cardiovascular system.  The body must be presented with a workload that challenges its current fitness state.  This workload will cause fatigue, and with proper recovery, will eventually yield cardiovascular improvements.  If the workloads are the right magnitude - slightly more than the body is currently used to then adaptation occurs.  It is important to note that the overload happens during the exercise while the adaptations occurs during recovery which makes recovery a vital part of your program.

Robbins goes on to say that the most important benefit of interval training is an increase in your metabolism.  Studies have shown that interval training raises your metabolism after a workout, and keeps it up longer than any other steady state workout.  Steady State exercise refers to any exercise that stays at the same workload for a long period of time.11

Over-Training
Body fatigue and loss of sleep are two signs of over-training.  Measuring your recovery heart rate can also give you insight into this problem.  If your recovery heart rate is not dropping as fast as it normally does, it could be a sign that you need to rest.  Take time out and evaluate your workouts: are you getting sufficient amounts of sleep, how is your nutrition, are you getting enough water?  Burning out is your body’s way of saying it is time to slow down and rest, listen to your body.