How do I get my run animation to play while i'm already walking?

I've been experiencing with my own custom animated sprint script and i'm having problems with the Animation starting when i'm running but not when i'm standing still. here is my method:

local UIS = game:GetService("UserInputService")
local Player = game.Players.LocalPlayer
local isSprinting = false

UIS.InputBegan:Connect(function(input)
    if input.KeyCode == Enum.KeyCode.LeftShift and isSprinting == false then
        isSprinting = true
        Player.Character.Animate.run.RunAnim.AnimationId = "http://www.roblox.com/asset/?id=5202688739"
        Player.Character.Animate.walk.WalkAnim.AnimationId = "http://www.roblox.com/asset/?id=5202688739"
        Player.Character.Humanoid.WalkSpeed = 25
        if input.KeyCode == Enum.KeyCode.W or input.KeyCode == Enum.KeyCode.A or input.KeyCode == Enum.KeyCode.S or input.KeyCode == Enum.KeyCode.D then
            local Track = Instance.new("Animation")
            Track.AnimationId = "rbxassetid://5202688739"
            local Anim = Player.Character.Humanoid:LoadAnimation(Track)
            Anim:Play()
        end     
    end
end)

UIS.InputEnded:Connect(function(input)
    if input.KeyCode == Enum.KeyCode.LeftShift and isSprinting == true then
        isSprinting = false
        Player.Character.Animate.run.RunAnim.AnimationId = "http://www.roblox.com/asset/?id=180426354"
        Player.Character.Animate.walk.WalkAnim.AnimationId = "http://www.roblox.com/asset/?id=180426354"
        Player.Character.Humanoid.WalkSpeed = 16
        if input.KeyCode == Enum.KeyCode.W or input.KeyCode == Enum.KeyCode.A or input.KeyCode == Enum.KeyCode.S or input.KeyCode == Enum.KeyCode.D then
            local Track = Instance.new("Animation")
            Track.AnimationId = "rbxassetid://180426354"
            local Anim = Player.Character.Humanoid:LoadAnimation(Track)
            Anim:Play()
        end     
    end
end)