You should check out doing an Aerobic Threshold (AeT) test and an Anaerobic Threshold (AnT) test. I believe there are details on the Uphill Athlete site. But you should absolutely determine your AeT if you’re trying to base build in zone 2 (180-age is a good place to start, but it is likely lower). AnT is useful as well for higher zone training. It’s a more demanding test than AeT but will give you a more practical upper bound for speed workouts than max HR.
