First you should understand how your body works. It is a perfect machine bent on adaptation, it adapts to everything. You expose yourself to harmful chemicals and it will do it’s best to adapt, filter and survive; if it becomes overloaded you feel the negative effects of that exposure. This may seem like a drastic example for an article on exercise but it’s not really; you see your body deals with exercise much the same way.
When you exercise your body adapts, it increases heart rate and respiration in an effort to supply more oxygen to the muscles doing the work. It will utilize stored glucose to produce more energy and when those supplies run short will look elsewhere for ways to create fuel. As muscle fibers reach work capacity and begin to exhaust they begin to break down chemically into waste products like lactic acid. Your body through circulation works diligently to clear this waste at the same rate at which it is created, when it’s unable to keep up you “feel the burn” strength lessens and you become fatigued. If you push through beyond the point of discomfort pain increases, as pain increases so does damage to the working tissue and it’s ability to protect and support the tissues around it. An injury becomes more likely the further you push as the body’s tolerance for any unexpected circumstance is compromised. (ex. Suddenly running on uneven ground.)
When you stop exercising your body repositions it’s resources to increase repair and new construction. Each time you push your physical limits during this “down time” your body will replace resources like glucose, repair damaged tissues and in an effort to be more prepared next time, make them a little better. This is the real magic of exercise, after every single workout with adequate recovery your body will in fact be a little different than it was before.
Now I know what you’re thinking, if I workout harder and longer I can cause more adaptation or change after every workout. In the most simplistic manner this looks to be true, but there is a factor of diminishing return. When your tissues become compromised or “damaged” beyond a certain point you greatly increase your need for recovery but this doesn’t yield greater adaptation, just longer recovery. Further understand that during recovery because abundant additional resources are needed for repair these resources must come from somewhere, both from consumed fuel sources and often other systems such as the immune system. This is why if you’re feeling under the weather and then complete a really hard workout you might find yourself sick within the next day or so.
When it comes to workout duration there isn’t a right or wrong answer, but here’s something further to consider. Effective workout duration is equally influenced by fuel consumption, ability, and efficiency or intensity.
What this means is that if you’re eating poorly in the case of consuming processed foods or not eating enough your effective workout duration will be shorter because of diminished incoming resources for recovery.
If you are unfamiliar with the performance of the selected exercises inefficiency will lead to faster physical exhaustion and quicker damage to the affected tissues, meaning your workout duration will need to be reduced.
Finally, and the biggest variable factor is intensity. The lower your intensity the longer your workout will need to be, the higher the intensity the shorter it will be. Now here’s where this get’s weird. Our body is so perfect that there is a variable also to intensity, in that our subconscious mind will regulate our perceived intensity. In an effort to explain think of it like this, if you plan to workout for 60 minutes and do so on a regular basis your body adapts to this, it will limit your ability to maintain a certain intensity to ensure that you can complete the 60 minutes. Your body, by regulating your intensity, basically ensures that you will be able to perform in some capacity for that duration. Inversely if you choose to workout just 30 minutes your body will regulate in the same way, because the duration is less it will begin to allow more effort to be expended in that duration knowing that it doesn’t need to preserve resources accordingly for survival.
This is a very general explanation but in the end it all comes down to our body’s basic function to ensure survival. It will sub-consciously regulate your ability to perform in an effort to give you the best chance of survival if this were an emergency situation it’s really about that simple.
So in the end there isn’t really a set duration anyone should workout, high-level athletes generally have developed metabolic systems to be able to survive longer training durations. The average adult will be notably less, which is why all of our exercise programs generally focus on a 30-minute duration. We know in terms of capacity this will be very effective for nearly everyone. People are always surprised to learn that in my peak training days for the nationals I still used to only train about 30-45 minutes, 4 days a week.
Hope this helps, more isn’t better, smart training for the best results!