Lactate is not bad. It is actually fuel, and you can clear it and use it if trained properly.

Lactate spent decades as the villain of endurance sport, the burning stuff that ends your race. That story is backwards. Lactate is fuel. A trained body shuttles it out of the working muscle and burns it somewhere else, and the whole point of threshold training is making that clearance system bigger.

The threshold is just the pace where production starts outrunning clearance. Below it you can go for hours. Above it the clock starts. Which means the single most useful number in your training is where exactly that line sits, and most runners let a wrist algorithm guess it for them.

Mine guessed wrong by a lot. The lab drew 1.5 mmol/L at 7:00 pace and 4.0 mmol/L at 5:25. My watch had my threshold at 5:59. Every zone I had built was 34 seconds per mile too slow, which is why threshold work had felt suspiciously comfortable for two years.

The fix is a real test, and they are easier to get than people think. A university lab, a sports med clinic, or a 300 dollar handheld meter and a patient friend. I wrote up what the test involves and how to book one, and the numbers page shows what it costs and where to look.