Sprint Button

LocalScript

local button = script.Parent
local Player = game:GetService('Players')
local sprinting = false

local function sprint()
    local player = Player.LocalPlayer
    if sprinting then
        sprinting = false
        button.Text = 'Sprint: Off'
        player.Character.Humanoid.WalkSpeed = 16
    else
        sprinting = true
        button.Text = 'Sprint: On'
        player.Character.Humanoid.WalkSpeed = 50
    end
end

button.MouseButton1Click:Connect(sprint)

Hold Button to Sprint

local button = script.Parent
local Player = game:GetService('Players')

local function sprint()
    local player = Player.LocalPlayer
    button.Text = 'Sprint: On'
    player.Character.Humanoid.WalkSpeed = 50
end

local function walk()
    local player = Player.LocalPlayer
    button.Text = 'Sprint: Off'
    player.Character.Humanoid.WalkSpeed = 16
end

button.MouseButton1Down:Connect(sprint)
button.MouseButton1Up:Connect(walk)