Hi Brian, Lactate threshold *, Threshold and Functional Threshold are all pretty much describing the same thing. Its that point during exercise where there's almost an equilibrium blood lactate removal and production.
If you ride above that threshold intensity you're gonna have to slow down after roughly 5-10 minutes. (hence you can blast up Box Hill above threshold but you couldn't keep that intensity going for much longer). If you ride below threshold intensity all thing considered equal you could keep the effort going for hours within (cyclosportivs, club run etc)
I've put an axterix next to * Lactate Threshold because by its true scientific definition its actually below Threshold/Functional Threshold or indeed Critical Power. Lactate Threshold intensity can be maintained for at least 2/3 hours. However this term Lactate Threshold has been misrepresented by most 'cycling coaches' so when people (not Exercise Physiologist or Sports Scientists) refer to Lactate Threshold they are actually referring to THRESHOLD
Depending on your fitness levels your threshold heart rate can vary between 84-92% of your max. Finally if it feels like this then you're doing a 2 x 20 at threshold Threshold intensity:
Slight/mild burning in the thighs, you're able to mutter a few sporadic words but not hold a full conversation, breathing is laboured, it takes a lot of concentration to maintain the effort
By the way, well done mate. I'll be watching this space carefully