How would this work using ContextActionService?

I'm new to CAService and keyboardInput in general, so how exactly would I go about executing part of a function once a certain key is pressed, (and holding) then when the key is released running the other half? So far I've tried running a portion of it when the UserInputState.Begin (the main animation), then the other portion when UserInputState.End (the idle animation). And for some reason it seems that only the first half runs.

local CAService = game:GetService("ContextActionService")
local deb = false
local function first(actionName, inputState, inputObj)
	if not deb then
		if inputState == Enum.UserInputState.Begin then
			deb = true
			local player = game:GetService("Players").LocalPlayer
			local character = player.Character
			local Animation = Instance.new("Animation")
			Animation.Name = 'Swimming';
			Animation.AnimationId = "rbxassetid://2130790796"
			local track = character.Humanoid:LoadAnimation(Animation)
			track.Looped = true
			track:Play()
			if track.IsPlaying == true and Animation.Name == 'Swimming' then
				character.Humanoid.WalkSpeed = 16
			elseif inputState == Enum.UserInputState.End then
				local Animation = Instance.new("Animation")
				Animation.Name = 'Treading';
				Animation.AnimationId = "rbxassetid://2130697426"
				local track = character.Humanoid:LoadAnimation(Animation)
				track.Looped = true
				track:Play()
					if track.IsPlaying == true and Animation.Name == 'Treading' then
					character.Humanoid.WalkSpeed = 0
					deb = false
				end
			end
		end
	end
end

CAService:BindAction("keyPress", first, false, Enum.KeyCode.Up)