Walking is the most important basic motion in Taekwondo. It must be so in every martial art. Admitting any martial art is a system of fighting skills, if you can control the distance between you and your opponent, almost every problem will be solved. Imagine what if you can approach your opponent and part from him as fast as you want at will. So, Distance is all of what is between you and your opponent(Ch.20 PPT). This walking is the very motion or skill of how to control the distance.
The essential point of walking in Taekwondo can be summed up in brief : to keep in low state during the walk and to delete every excresent(i.e. unnecessary) motion. The following pic shows a good training method for TKD's walking.
As you see, the point of this training is walking in low pose. Here, you should not drop your shoulder low as beginners do usually. If you keep your hip high with your shoulders low, you will come to feel hardness on your waist rather than on your legs. You can see some bad poses compared with better ones here.
The left pose is better than the right one in each pic. In first pic you can see the right man spread his arm forth. It means he cannot balance without his arms since his hip is more backward. (See Ch.3-4 of 3JKY)
In the second pic you can see the left man is standing on two legs actually though it is hard to see out. You can see his upper body is slightly bent down forth too. You may be able to see his hip is pushed back more than good one in the third pic.
But it would be almost impossible to immitate the better pose as a beginner. Then don't oppress yourself too much, but just try this exercise much time. You can improve your pose only in training, not in thought.
You should straighten up your upper body with only your hip low, as low as the height of your knees. It is said that the sky is high while the earth is low, where your head corresponds to the sky while your hip to the earth. (You will see some more of this kinds of teachings in On Samjae & Kang-Yu.)
The next point of this training is standing low on only one leg. Observe each pose. The man is standing on only one leg in each step except for beginning and ending. (You cannot see the ending pose here) In order to practice this training you should strengthen your legs in more basic training of stand-up. You should be able to walk as calmly and slowly as possible.
The third point is related to breath. But it is difficult to explain reasonably.
You should be able to walk this way at least for 3 minutes.