Training zone 2 is your all-day endurance pace: 59-75% of your Ftp (Ftp = Functional threshold power) and is extremely valuable for your training. Being able to ride 2-3 hours in Zone 2 is the first step in an athlete’s training for being able to compete in races or events of similar duration. Also “heavy” training with friends. By riding in zone 2, athletes can train their body to burn fat in preference of muscle glycogen.
Substrate utilization (i.e. fat vs. carbohydrate) varies individually, but as a rule of thumb, as intensity increases energy supply from carbohydrate increases, while supply from fat decreases. Therefore it is important to not go too hard in zone 2 training. For long zone 2 training rides, often the magic and ‘fat adaptation’ doesn’t begin until the last hour of the ride (hour 3 of a 3-hour ride).
An endurance athlete should never stop training in zone 2. The ideal training plan should include 3 days a week of zone 2 training in the first 2-3 months of pre-season training, followed by 2 days a week as the season gets closer and 1-2 days of maintenance once the season is in full blown.