Blend Trees blend multiple animations based on the chosen Animator Controller parameter value and Motion fields' thresholds. In the standard Blend Tree, always only two animations are blended at the same time: the animation with a lower Threshold and the animation with a higher Threshold. Each animation is blended with the weight corresponding to the distance between the actual parameter value and the Threshold of the animation. For instance, if your Walk animation Threshold is set to 1, your Run animation Threshold is set to 2, and your current Speed parameter value equals 1.5, both Walk and Run animations will be played with 50 percent weight, which will most likely result in your character jogging slowly.
When blending Walk and Run animations, make sure to have the contact poses in the same normalized time in both animations. For instance, if the Walk animation has contact poses in 0 percent, 50 percent, and 100 percent of the animation, the Run animation should also have contact poses in 0 percent, 50 percent, and 100 percent of the animation. That will assure proper and smooth blending between the animations.