To do this you want get velocity vector length <= walk speed from walk speed blend and if you want it to really work base it of the forward vector that looks good. Do this by hooking up a print string to show the vector length. Pay attention to the variable when you feel like your character should begin using stamina. Set <= bottom input as the number you found. I recommend changing your run speed animation to a speed that can accommodate your or making the character use stamina at the same speed the character begins to lean forward in the idle walk run blend space.