Where does lactate fit into the picture, Greg Brock just nails it:
There’s even a physiological correlative to this when you push into oxygen debt, you back off just a little bit, your body learns to utilize the lactic acid for fuel, lactic acid plus oxygen is pyruvic acid and guess what? – we are right back in the Krebs cycle again. What happens is your body learn to create more enzymes that speed up that reaction of lactate back to pyruvate and when you do workouts like that, that’s why it’s so effective.
Why do we keep preaching that lactate is bad? It is a fuel, learn to metabolize it and make it your friend. Read Tim Noakes, Lore of Running for more on the role of lactate. Oh by then way it is not lactate that causes soreness. That is another issue for another time. Read the Noakes book and it will expand your vision.