How do I use a toggle to switch the default walking animation?

I'm making a script, it uses a toggle animation system. What's really annoying is that I have to let go of W for the animation to actually switch.

What I do in the script is I take an animation ID and I switch the default walk animation to that animation. That animation keeps playing until I let go of W to start walking again. Here is a small bit of my script.

    Crouch:Play()
        Character.Humanoid.WalkSpeed = 8
        Character.Animate.walk.WalkAnim.AnimationId=script.CrouchWalk.AnimationId
        wait(2)

What happens is there the idle animation plays, and does the walking animation. But until I let go of W or whatever I get the default one. If anyone could help that would be great, thanks!!