I keep putting out the term VO2max but what is it?
It's a measure of the maximum rate your body can consume oxygen for a specific activity (in our case, running).
As you run faster and faster you approach a point at which you are no-longer running aerobically (with oxygen) and reach a deflection point where you are running anaerobically which requires your body to start using fuel sources other than Oxygen. This point is knows as your anaerobic threshold. If one runner has a VO2max that allows him to continuously run 5:30 miles for a 5k race, and another can only manage 5:35 per mile before going over his limit, which one do you think will win? Every thing else being equal the first runner with have a 15 second advantage. Obvious the higher your VO2max, the better your advantage when racing. You may be able to outrun someone with a lower VO2max in a really long race that's completely aerobic just by putting a lot of long slow miles under your belt. But in a 5000 meter run you also need to raise your VO2max so that you'll be able to hold the required pace of a 5k more comfortably. There have been many studies, both scientific and empirical, on the best way to do this and it's universially agreed that the best way is to train at or a little above the pace where you encounter your current VO2max.
The other type of threshold run we do is called the Tempo Run. A tempo or threshold run is at a slower pace than the VO2max runs but they're generally longer. OK... but why do we do them? We do them to increase the running speed where we start accumulating lactic acid in our muscle cells. As we increase speed, not only must our cardiovascular system supply oxygen to our muscles but it must also remove waste products.., in this case, lactic acid. As speed increases, the build-up of lactic acid is very slow until suddenly a zone is reached where it increases vary fast. This zone is called your lactate threshold. When this happens your heart-rate goes up very quickly and you will have no choice but to slow down, stop, or struggle in. As with anaerobic threshold, the best way to increase the velocity where this will happen is to train at speeds in this zone. This is what we call tempo or threshold pace. It's about the pace you can hold for an hour of continuous running. Tempo running, by the way, has many other benefits and is always an excellent choice if you don't know what kind of run to go on on any given day.