Run Script Once:
_G.WalkSpeed = 150
local run = game:GetService('RunService')
local players = game:GetService('Players')

if _G.InitWS ~= true then
	run.Heartbeat:connect(function()
			players.LocalPlayer.Character.Humanoid.WalkSpeed = _G.WalkSpeed
	end)
	_G.InitWS = true
end

To Change Speed Afterwards:
_G.WalkSpeed = [ENTER SPEED HERE]