For as long as people have cared to try to increase their cardiovascular capabilities the advice has been pretty much the same. Perform semi-regular medium to long distance runs, rides or swims.
However, this will not prepare you for short bursts of intense movement. The ability to run a slow 5k will never help you perform well at the 40 yard dash. Conversely the short intense bursts of cardiovascular work will pay great dividends toward medium and long distance events.
This is the great failure of traditional fitness classes with set time parameters. If all you ever do is 30, 45 or 60 minute classes you will be fairly good at doing those types of things. Put a light sandbag on your shoulder and run uphill for 150 yards and you will quickly find out you're not as fit as you once believed.
Training the 3 metabolic pathways
1. Phosphagen: short burst max efforts 20 seconds or less.
2. Glycolytic : mid range intense work performed between 2- 20 minutes in duration
3. Oxidative: 20 minutes of steady state cardio work performed at 65-75 % of your max heart rate.
If you constantly vary the time domains and then also constantly vary the type of tools used, a very high level of fitness can result. Instead of routinely running 3-5 miles a day, try accomplishing as many rounds of 5 pull ups, 10 push ups and 15 bodyweight squats as you can in 10 minutes. You'll be shocked by the difference in effort it takes and the challenge it represents to your cardiovascular system.
Remember... routine is the enemy of fitness.
For more info on the benefits of CrossFit training contact your local CrossFit gym.