Lactate is produced in muscles, then some is then metabolized in other muscles, liver etc. If the exercise is intense enough, the production rate exceeds the clearance rate. Thus, some accumulates and that is what one measures with blood concentration (e.g. 4 mmol).
To calculate the rate of lactate production, you need to use radioactive tracers (i.e. labelled) so as to calculate rate of lactate appearance/disapperance in the blood, you also have to measure blood volume using one of various techniques (thermal dilution, green dye etc.). In short, very expensive and invasive. If you want to do these experiments, Prof. George Brooks at UC Berkeley is the man to see.
I am not aware of any such experiments involving fast running. Most experiments are on a cycle erg because you don't want to mess with the catheters going into the femoral artery & vein.