The example that the orginal poster stated about going out too fast in the first few miles of the half marathon is a prime example of your body’s ability to NOT recover from incurring too much lactate.
This is why a half marathon is a great race and a dangerous one to boot. Because it should be run right about at 100% of your LT pace. It is like walking a tight rope, lean one way too much and you stray off course.
But because he could not clear lactate any more, after plunging way to far into above LT range, he found himself accumulating more and more lactate. Even slowing his pace a little didn’t help. The damage was done.
With a HRM he would have seen this as eveidence but certainly didn’t need it to know that.
When learning to use the HRM he would’ve known that the first few miles of the half are soooooo important to stay on pace. But his HRM would not have climbed to the zone where he wanted to be until about the 4th mile.
But by 4 or even 5 he might have seen a HR way above the target and may have been able to stave off a little bit of the fading per mile paces. By 4 or 5 he might not have felt like all was lost yet. This can come with running more half marathons and or other races.