Lactate Threshold (LT)  is when lactate builds up faster than it can be flushed out of your blood.  When lactate builds up, you start to shut down.  So your body must become more adept as flushing that and other toxins from your body in a more efficient fashion.

The bottom line is... Training Hard raises LT!  If you want to get faster over longer distances, then you need to raise your lactate threshold. 

You may hate doing speed work and long intervals, but it is a necessity if you want to run faster times.

Active has a closer look at LT and VO2 Max, and how to increase both.