For many people who are used to traditional approaches to physical fitness such as weight training and running, this can be one of the more difficult notions of Tai Chi practice. I think that the key here is to realize that Tai Chi asks you to try a completely different way of activating and accessing the muscles systems of the body. In doing so, you must change your mental attitude and learn to "unclench your muscles" and "let go."
Use the mind instead of force.
In the "Tai Chi Chuan Treatise," it was said the one should use the mind instead of force. While you practice Tai Chi, you should relax your entire body without using any clumsy strength to leave in your tendons, bones, and blood vessels. The strength will only tighten up your body. Without using strength, you can change your steps with agility, lightness, and turn easily. Some people may hesitate with the thought of not using force, wondering how to increase human strength.
...
It was said from the "Tai Chi Treatise", "extremely soft, then extremely hard and strong." If one masters Tai Chi Kung-Fu to a high level, their arms and shoulders will be like steel wrapped with the appearance of cotton (hidden extreme power). It is a big difference from those who practice some "outer academy". While they are using strength, they have strength. If they don't use strength, it is like floating. You can see that "outer force" does not last long, and it's not released from the internal body's jing."
For me, there is not an exclusive, either or, relationship between Tai Chi and "external methods." It is certainly neccessary to organize one's training so that the "external" methods do not interfere with learning the "internal" methods of Tai Chi. It is also important to realize that most of us have spent most of our lives devoted to "external" methods and have entirely neglected the "internal" approaches.
Keep these things in mind:
Do your Tai Chi training before weight training or intense aerobics activity or on alternate days. These "external" activities, while generally good for you, will directly interfere with your ability to settle, relax and let go, if done before your Tai Chi session. On the other hand, Tai Chi will warm up your body and settle your mind. I find this improves results from strength and aerobic work.
Over time, try to increase the time you devote to Tai Chi practice (or other mind body practices if you prefer). The mind body link will likely have been, the most neglected and therefore the weakest aspect of your physical training throughout your life, and it always pays most to work on weaknesses first, rather than continue to do the familiar and easy.