I think one of the ideas behind the recommendation for "conversational exercise" is to remain aerobic.

Exercise requires increased power output, which requires that your body converts "air and oats" to muscular energy at an elevated pace compared to couch-surfing. Depending on your fitness and conditioning, you have a level which is the capacity of your body to achieve this energy conversion. Below that level you can take in enough air to fulfill the power output requirements, and your breathing is essentially normal. Deeper and faster perhaps, but in a mode you can sustain for a good length of time. Above that level you are consuming and processing more oxygen than you can take in and process, and panting and extreme breathing results as you go into oxygen debt. The recovery time of aerobic exercise (body must recover from the stress, this is what makes you stronger) is much shorter than for anaerobic. The ability to ride longer distances, like a 100 km or 100 miler or really anything more than a few miles, is based on efficient and strong aerobic riding, not extended anaerobic riding.

Power at lactate threshold would seem to be a more significant measure for a recreational rider than say, short-term anaerobic power. For a strong rider the former could be maybe 200 watts, and the latter a lot higher for very brief efforts.