Lactate as fuel rather than a waste product
The older idea that lactate is a metabolic waste that simply causes fatigue has been largely revised. Lactate is continuously produced even at rest and during light activity, and it serves as a readily usable fuel. The lactate shuttle concept, developed largely from the work of George Brooks, describes how lactate moves between and within cells to be oxidized for energy where it is needed.
In practice, fast-twitch fibers that rely heavily on glycolysis produce lactate, which can then be taken up by slow-twitch fibers, the heart, and other tissues and converted back toward pyruvate for oxidation. Transport relies on monocarboxylate transporters in cell membranes.
Why blood lactate rises during hard exercise
Blood lactate climbs during intense exercise when production outpaces clearance, not because muscles have run out of oxygen in a simple sense. The acidity associated with hard efforts comes from several coupled processes, and lactate itself is better viewed as a marker and a fuel than the direct cause of the burn.
This is general educational background on exercise physiology, not medical or training advice. Lactate threshold testing and pacing decisions for athletes are best interpreted with a qualified coach or exercise physiologist.