Scientific Triathlon has a great podcast. As an example here is a past episode on the topic of fueling for a triathlon, with transcription:
It is not specifically about training at lactate threshold.
Alan Couzens has a blog and frequently posts about training zones on Twitter. For example lactate curves for 2 pro triathletes with one training upper z1 and the other upper z2:
A lot of food for thought for IM training. You may not agree with him, but reading the posts and discussion is interesting and thought provoking. And his website: https://www.alancouzens.com
As a result of following him, I’ve started following others on Twitter, for example
those are off the top of my head, hope that helps.