There are a variety of scales to measure training zones, and it can be confusing. Here is a quick primer on energy systems and muscle fibers, which covers the most prevalent researches:

EASY:Generates energy from fat, and builds endurance.

STEADY: Builds aerobic energy. Primary racing zone for IM age groupers. Benefits include retraining of Type 2a muscle fibers. MOST Ironman triathletes spend too little time training here and have poor capability in this zone.

MODERATELY HARD:Enables more intense sets. This is absolute max aerobic for the very best trained Ironman athletes.

HARD: Threshold training. Enhances VO2 max.

VERY HARD: Decreases aerobic power.

Muscle fibers
Type 1 – Slow twitch – Pure oxygen burners (aerobic); primary movers for endurance.
Type 2a – Fast twitch – Fat oxidative. These fibers can be trained to go either way. Very beneficial for endurance athletes.
Type 2b – Fast twitch – Pure glucose burners (anaerobic).

Anaerobic stress produces high level of cortisol which impairs aerobic function and fat burning. Most Ironman age groupers experience this. A slow aerobic pace indicates that an athlete’s muscles are more developed than his/her metabolism/aerobic function. First we train the metabolic process to handle the task; strength, speed and endurance will improve.