Stress

Many runners have careworn faces with deep lines caused by emotional stress.
They don't look happy when running.
They look deeply upset.
Their approach to running is causing them to suffer.
Over-training, pushing the body and pursuing unnecessary goals is harmful and leads to pain and injury.
There are other ways to run and/or to get fit.

Is your running healthy?

Some runners can run well. 
Most do not.
If your aim is to get fit, then you want to avoid medically unsound running practices.
Usually, people take bad habits of body use from everyday life straight into their running...

Running well?

The fact that you are running isn't proof that you are running well.
By that logic, couldn't any individual with no tuition whatsoever drive a car immediately or play the piano?

Quality of exercise

Most people who run have received no formal training in how to run.
They just start running.
Little or no thought is given to technique, alignment, good body use, breathing, skeletal health, muscular tension, coordination, accuracy, biomechanics or suitability.

Is running easy?

A lot of websites and articles indicate that running is easy to do.
This is an interesting assertion; suggesting that virtually anyone can just buy a pair of running shoes and hit the streets.
Of course, this is true but it doesn't answer the question.
Is running easy to do?

Hard work

Running is hard work.
If you persist with running and enjoy the experience, the sensation of running and the endorphin can make running fun. 
Ultimately, it depends on the individual - we all like different things...

Boredom

If you look at a popular form of exercise such as running, it is easy to see why people don't usually exercise.
Running is not mentally complex or physically sophisticated.
It essentially involves doing the same thing again and again and again.
This is why people often wear headphones and play music: they are bored.

How

It is not just a matter of exercising. 
How you exercise matters too. 
If you exercise without professional guidance, then habits of poor awareness and bad postural/alignment will persist.
The world is filled with well-meaning runners who exhibit appalling physical tension and deleterious poise.

Is tai chi compatible with the gym?

Gym work typically shortens muscles and promotes patterns of tension and fixity in the body.

Tai chi does the opposite: it allows muscles to lengthen, releases tensions and encourages good skeletal use.
The muscle work that takes place in tai chi is principally geared towards whole-body movement and training the body to rely more upon postural muscles.
 

Body building and weight lifting are fashionable activities today. The emphasis is upon developing external muscles which creates an armouring effect that can eventually distort the bony structure. It is the over developed musculature that actually torque's the bones and discourages them from bearing additional weight. The body attempts to compensate and problems arise.

Running does not necessarily in and of itself improve posture that is already poor and constricted. It often exaggerates problems due to the substitution of inappropriate muscles. The repetitive inappropriate development of the musculature (as in body building or weight lifting) often leads to diminished sensitivity. Stress occurs in the knees and lower back, encouraging injury.

Various sport activities emphasize strength, endurance and speed. Development of muscle control rather than skeletal balance takes precedence. Gaining speed at the expense of mounting tension, is too often the goal. 

(Liz Koch)

The health benefits of tai chi are quickly undermined by weight lifting, exertion and forcing.

Bad posture

Many runners run with extremely bad posture.
Their elbows are stiff and locked (often raised)... shoulders are lifted... the body is stooped... and weight is bearing heavily down into the knees.

This would be bad posture for any form of exercise.
It causes serious fatigue and adverse wear and tear on the body.