I was wondering how you have determined your Aerobic (AeT) and Anerobic (AnT) Thresholds ?
Zone 3 is pretty intense and a long (few hours) effort in zone 3 is hard to sustain.
If you’re able to sustain zone 3 for a few hours on a long hike or a long bike ride it might be you’ve underestimated your AeT and/or your AnT.
As for training both systems, sustainable and successful training at higher intensities happens when you have a strong aerobic base. Given a finite number of hours in which to train you’ll be most successful if you spend those hours developing the aerobic base first and then add intensity.
The warning from TftUA p. 92: “For those with insufficient aerobic base development, as indicated by the 10% test, overuse of this intensity [z3] can lead to a performance decline.”