Your VO2 max (Volume, Oxygen, Maximum) is probably the best indicator of your aerobic fitness and increasing it will almost certainly make you a better athlete. So what is it? In short, it is the exercise intensity at which your body reaches it's maximum volume of oxygen consumption. Usually if you run 1500m to 3000m at race pace (the fastest pace you can sustain for the distance) or the equivalent in cycling or swimming you will be operating at your VO2 max. The greater your VO2 max level, the more oxygen you can get to your muscles, and the faster or longer you can swim, cycle or run. That's my understanding of it anyway.
So, how can we increase it? Consistent steady aerobic training is an effective way to increase your Vo2 max. However, once you have built a strong aerobic base, running long intervals (3- 5 minutes with 2-3 minutes rest) at a pace that forces your body to perform at it's VO2 max can be introduced. These intervals force your body to adapt and improve as a response to the stress of the training. That's the idea anyway.
Here are some workouts that I will be trying in my training. All sessions should take less than or about an hour to complete depending on your ability.
Running: WU, 2000m easy. MS, 6 x 1000m race pace interval with two minutes rest. WD, 1000m easy.
Swimming: WU 400m easy focusing on technique. MS, 8 x 200m fast pace with 90 seconds rest. WD, 100m easy.
Cycling: WU, 7k easy graduating to moderate pace. MS 6 x 2k fast pace with 2 minutes rest. WD 7k moderate graduating to easy pace.