According to T Nation, walking does not build muscle. While it has a host of benefits, weight training should be done when attempting to gain muscle.
T Nation states that walking is an exercise that is great for burning fat. It is considered a low-intensity exercise and burns a higher percentage of fat. Further, in most instances, walking burns no muscle.
However, when on a diet and lifting weights, cardiovascular exercise causes the body to release cortisol. This hormone helps convert amino acids into glucose and is used to fuel the body. If the body has little fat stored, then the body may end up converting amino acids from muscle tissue.