Jack Daniels' book about running has a training plan that seems to be highly regarded.

• LT1 - 75% of maximum heart rate. ~145BPM maybe (for running).
• LT2 - Average heart rate for a maximum sustained effort for 1 hour.

Zone 1

• Trains your body to use fat stores as energy.
• Increases your oxidative capacity.
• Trains slow-twitch muscle fibres.
• You can speak a full sentence without taking a breath.

Zone 3

• Trains your body to use carbohydrates as energy.
• Trains your glycolytic system.
• Increases your ability to mitigate lactic acid.
• Trains fast-twitch muscle fibres.
• You can barely speak in this zone.

Training

• Zone 2 is not a good place to train, it's middle of the road and not an efficient use of time or effort.
• 80% of training should be in zone 1.
• We have much more energy stored as fat than we do as carbs.