How to make ALL players speed up?

I'm creating an obby that you have to go fast. I got a solution, but i had my friend try it and it won't work for him. And i can understand why, because this is what i used.

game.Workspace.Roblozinko.Humanoid.WalkSpeed = 75
repeat
	game.Workspace.Roblozinko.Humanoid.WalkSpeed = 75
	wait (.5)
until 6 == 5

It's because his and my username is different, so it only works for me. and i tried one from another question,

wait (5)
repeat
	game.Workspace.Player1.Humanoid.WalkSpeed = 75
	wait (.5)
until 6 == 5

and that didn't work. I got the output, "Player1 is not a valid member of workspace." I also tried it in a localScript and Script. Please help! thanks. :3

Maybe try this method.

local Players = game:GetService("Players")
for _,Player in pairs(Players:GetPlayers()) do
	local Char = Player.Character or Player.CharacterAdded:Wait()
	if Char then
		Char:WaitForChild("Humanoid").WalkSpeed = 75
	end
end)

x = 50 --speed boost

for i,v in pairs(game.Players:GetPlayers()) do v.Character.Humanoid.Walkspeed = x end