If your cardiovascular endurance is already high, but you wish to improve your performance levels, then its time to challenge yourself with serious interval training. Medium Interval Training aims to increase your VO2max capacity, through interval bouts. It should be performed at an intensity of about 90-95% of your VO2max, or 95%-98% of your MHR, maximum heart rate. Be warned, this is extremely demanding, and you need to be well conditioned for this type of training.
Make sure you warm up and cool down properly, 5min each, to get the most out of you training, and avoid injury. You should perform 1-2 series of exercise, with 4-5 repetitions per series. Each repetition should be sustained for 2-4min. Rest is important here, so take 50-100% (relative to the length of your repetitions) rest time in between each repetition. Your perceived effort level for this type of training should be in the 6-7 range. Due to the high level of demands this training method places on your body, 36-48hrs will be required to recover properly from this.
Next week, Short Interval Training.