Bonus Solution

local player = game.Players.LocalPlayer
local hum = player.Character:WaitForChild("Humanoid")

local WALK_SPEED = 16
local INCREASE = 0.1 -- Constant increase in acceleration
local INTERVAL = 0.1
local acceleration = 0 -- Acceleration is now a variable, changebale, not a constant

while true do
    while hum.MoveDirection ~= Vector3.new() do
        acceleration = acceleration + 0.1 -- Increasing the acceleration
        hum.WalkSpeed = math.min(hum.WalkSpeed + acceleration, 250) -- Limits it to 250
        wait(INTERVAL)
    end
    hum.WalkSpeed = WALK_SPEED
    hum.Running:Wait()
end