Lactate threshold
Lactate threshold occurs when the metabolites of exercise are not able to be buffered. Chiefly, during anaerobic glycolysis, hydrogen ions accumulate faster than they can be removed (buffered). Muscles start to burn!
Running fast has some specific benefits that make it worth the effort.
VO2 max will move up a bit.
Lactate threshold will improve.
There is good evidence of improved cognitive function as a bonus!!
Most runners on a comfortable training run can carry on a conversation with some difficulty. Great for building endurance. You're probably running at 50 - 65% of your VO2 Max.
Run your fastest 5K. You're at about 95% of your VO2 Max.
Run a marathon. 80% of your VO2 Max.
Your best 10K. You're probably running at 90% of your VO2 Max. This is also about your lactate threshold.
Here's the 30 minute workout:
Run at a conversational pace 5 minutes (a bit slower than a marathon pace). Would be able to hold conversation at this pace.
5 x 2 minutes at a hard 10K pace. Breathless but not exhausted. 2 minutes of conversation pace between these 5 intervals.
Cool down for 5 minutes at slow conversational pace.
Don't do it too often, perhaps once every two weeks. More than that and your risk of over-training probably outweighs the benefit.
Happy Running!