This is a very uncommon way of starting animations, which works but is very limited.
You can use the following code instead, if you need to change transition times via code:
Spine.Unity.SkeletonAnimation skeletonAnimation;
var trackEntry = skeletonAnimation.AnimationState.SetAnimation(0, "Walk", true);
trackEntry.MixDuration = 0.3f;
Alternatively, you could also set the mix durations at the skeletonData asset, which is the recommended way to setup animation transition times.