For TxRunnerGirl

This is from an article entitled Training Intensity and Heart Rate by Eric Schwartz.

Heart Rate Zone	Effort	Description
Zone 1	Easy	Recovery. Endurance building. Conversational pace. You can hold this pace for a long time. Commonly referred to as aerobic training.
Zone 2	Easy to Moderate	Extensive Endurance. Endurance building. You can usually hold this pace a long time. On days after hard workouts zone 2 may be a little more difficult and it may be best to avoid in this case. Commonly referred to as aerobic training.
Zone 3	Medium	Intensive endurance/muscular endurance. Beginning athletes can maintain this pace for 15-40 minutes, while experienced athletes may be able to hold this effort for 90+ minutes in training.
Zone 4	Medium Fast	Sub-threshold. Your ability to maintain this pace in training will range from 20 minutes to 2+ hours.
Zone 5	Fast	Super-threshold. Lactate threshold pace. This pace can be maintained in training for 15-60+ minutes depending on fitness.
Zone 5b	Very Fast	Anaerobic Endurance. This pace will be very difficult to maintain for more than 15 minutes. Breathing will be very hard and you may last as little as 2-3 minutes at this effort.
Zone 5c	Very, Very Fast	Power