Summarizing the workouts you linked to, I see:
- ~30 minute anaerobic test suggests AnT at ~192bpm, 7:46/mi pace
- 1hr aerobic workout, 164bpm, 10:11/mi pace, 2.67% drift
- 1hr aerobic workout, 170bpm, 9:32/mi pace, -0.74% drift
- 1hr aerobic workout, 183bpm, 9:18/mi pace, 1.22% drift
I’m guessing that you may have had slightly higher drift at 185bpm (i.e. your last workout where the chest strap cut out), and that you could conservatively consider your AnT to be 190bpm, AeT to be 180bpm.
You might consider making Z4 to be >190bpm, Z3 180-190bpm, Z2 165-180bpm, Z1 140-165bpm, and target spending more time in Z1 than anything else.
With 600 yearly hours of training, it seems like you might have opportunity to increase strength / force production and increase both your AnT and AeT speed, depending on your training goals. If your AeT speed on a relatively flat treadmill increased to an 8:00/mi or 7:30/mi pace, that training might also get you closer to 4mph or 5mph for an ultra.