I know the only way to determine your exact lactate threshold is through lab testing....My question is this:
In the 2x20 minute test to estimate threshold heart rate, isn't the duration of the test too short? I mean surely even well-trained athletes can sustain higher heart rates for 20 minute intervals than they could sustain for an hour or more.

On the other hand, whether my training zones are accurate or not, at least I know my training is working. I just did a graded exercise test after 8 weeks of training and I lasted 17 minutes compared to 16 minutes previously, and my maximum power increased by 20 watts. Joy!!!!