Hi Kira,
Sorry for the late reply.
In basic terms, in the Uphill Athlete methodology, these are the zones:
Recovery Zone: ambling along a flat street, very relaxed
Zone 1: easy effort, you can easily carry on a conversation without puffing
Zone 2: harder effort but you can still talk in complete sentences : Aerobic threshold (AeT) = top of Zone 2
Zone 3: “fun hard” — an intensity you can maintain for at most one hour : Anaerobic threshold (AnT) = top of Zone 3
Zone 4: hard – an intensity you can maintain for at most several minutes
The UA method uses the two threshold markers — AeT and AnT — to guide training.
This article outlines the zones and shows their relationship to the metabolism (using fat vs using carbs/glucose for fuel):
This article explains (with a video) how to do the HR drift test to determine the top of your zone 2: https://uphillathlete.com/aerobic-training/heart-rate-drift/
Does that help?
Jane