For me LT doesn't come into it. Everything is based on incremental change to intensity or duration of pain, coupled with liberal doses of rest.

Since I can't afford a power meter the intensity is based on heart rate zones. The zones are relative to maxHR and are set based on the results of vomitous tests which were used to estimate my LT in the absence of a lab.

Now HDT is on the right track because LT will change with training, so you have to retest periodically to keep your training zones in check. NSWIS recommend once every 3 months, otherwise their lab techs sit around with nothing to do. At mug level like me, once every 6 months is enuf.