Stamina/Sprint function not working properly? Any Ideas?

Alright so I have this stamina/sprint (local) script and its not preforming like I expected... Instead of it actually letting you run, it doesn't.. 1. the while true do loops only runs once? 2. It wont let you run and stop running?

local InputService = game:GetService("UserInputService")
local Player = game:GetService("Players").LocalPlayer
local Character = Player.Character or Player.CharacterAdded:wait()
local Humanoid = Character:WaitForChild("Humanoid")
local IsSprinting = script.Parent.Variables.IsSprinting
while true do
    if(IsSprinting.Value == false) then
        if(script.Parent.Variables.Stamina.Value < 100) then
        wait(.25)
        script.Parent.Variables.Stamina.Value = script.Parent.Variables.Stamina.Value + 1
        script.Parent.GUI.Stamina.Text = "Stamina: "..script.Parent.Variables.Stamina.Value.."%"
        return
        end
    end
end
while true do
    if(IsSprinting.Value == true) then
        if(script.Parent.Variables.Stamina.Value < 0) then
        wait(.05)
        script.Parent.Variables.Stamina.Value = script.Parent.Variables.Stamina.Value - 1
        script.Parent.GUI.Stamina.Text = "Stamina: "..script.Parent.Variables.Stamina.Value.."%"
        return
        end
    end
end

InputService.InputBegan:connect(function(Input)
    if Input.KeyCode == Enum.KeyCode.LeftShift then
        if script.Parent.Variables.Stamina.Value > 0 then
        IsSprinting.Value = true
        Humanoid.WalkSpeed = 25
        end
    end
end)

InputService.InputEnded:connect(function(Input)
    if Input.KeyCode == Enum.KeyCode.LeftShift then
        IsSprinting.Value = false
        Humanoid.WalkSpeed = 16
    end
end)