Script keeps running after I stop holding Shift?

I'm trying to make a running script where every 2 seconds of you holding Shift your WalkSpeed goes up by 1 until it reaches 25

After I stop holding shift the animation ends but it keeps going up until 25 even though I'm not holding Shift

local p = game.Players.LocalPlayer 
local char = p.Character 
if not p.Character then 
    repeat wait() until p.Character 
end

local human = char.Humanoid 
local anim = game.Workspace:WaitForChild("Animation")
local loadAnim = human:LoadAnimation(anim) 


local UIS = game:GetService("UserInputService")

local holdingShiftKey = false


UIS.InputBegan:Connect(function(inputObject) 

    if(inputObject.KeyCode==Enum.KeyCode.LeftShift)then
       holdingShiftKey = true


        while holdingShiftKey do
            loadAnim:Play()

            repeat wait(2)
                char.Humanoid.WalkSpeed = char.Humanoid.WalkSpeed + 1
                until char.Humanoid.WalkSpeed == 25
        end

    end
end)


UIS.InputEnded:Connect(function(inputObject)

    if(inputObject.KeyCode==Enum.KeyCode.LeftShift)then
                holdingShiftKey = false
        char.Humanoid.WalkSpeed = 16
        loadAnim:Stop()
    end
end)

Does anyone have a fix for this?