Sunday programming is focused on cardio respiratory endurance. Both the workout and the programmed run is intended to focus specifically on the development of aerobic capacity. 

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Cardio Respiratory Endurance.
‍Cardiovascular Endurance training is often referred to as Long Slow Distance (LSD) or Zone 2 (Z2) training. It is performed at low intensity - essentially the point where you can still comfortably hold a conversation. This lower intensity allows for great volume of work, which increases capillary and mitochondrial densities in your muscles, increases in the strength and size of your heart muscle, increases blood plasma volume, improves endurance (you can go longer before getting tired), burns fat, and increases your ability to recover. It is easy to push zone 2 training sessions a little too fast. It is better to be on the too slow side as opposed to too fast. These runs shouldn't lead to high levels of local muscle fatigue and/or soreness, but more of a general overall fatigue. The longer runs should leave you a little sleepy and hungry, but not beat you up. We choose to do our Cardio Endurance training once per week on Sundays, and we choose running because of the carry over to real life - self-powered, bi-pedal movement across the earth (but you can use a bike or rower if you prefer). If you have 30 minutes to train you will do a 2-3 mile run. If you have 1-2 hours to train on a Sunday you will climb in distance for three weeks, then reset back one week. This “wave progression” allows you to add volume without physically or psychologically overloading.