local UserInpServ = game:GetService('UserInputService')
local ReplicatedStorage = game:GetService('ReplicatedStorage')
UserInpServ.InputBegan:Connect(function(Input)
	if Input.KeyCode == Enum.KeyCode.E then	
		game.Players.LocalPlayer.Character.Humanoid.WalkSpeed = 72
	else
		game.Players.LocalPlayer.Character.Humanoid.WalkSpeed = 25
	end
end)
-- help

Formatting your code

Firstly, I formatted your code in a code block for you.

local UserInpServ = game:GetService('UserInputService') 
local ReplicatedStorage = game:GetService('ReplicatedStorage') 

UserInpServ.InputBegan:Connect(function(Input) 
	if Input.KeyCode == Enum.KeyCode.E then 
		game.Players.LocalPlayer.Character.Humanoid.WalkSpeed = 72 
	else 
		game.Players.LocalPlayer.Character.Humanoid.WalkSpeed = 25 
	end 
end)

The issue

Basically, you need to check if the keycode was E when the input ends, which will detect that it was held and you can set the walkspeed back.

Here is a fixed script

local UserInpServ = game:GetService('UserInputService') 
local ReplicatedStorage = game:GetService('ReplicatedStorage') 

UserInpServ.InputBegan:Connect(function(Input) 
	if Input.KeyCode == Enum.KeyCode.E then 
		game.Players.LocalPlayer.Character.Humanoid.WalkSpeed = 72 
	end
end)

UserInpServ.InputEnded:Connect(function(Input)
	if Input.KeyCode == Enum.KeyCode.E then
		game.Players.LocalPlayer.Character.Humanoid.WalkSpeed = 25
	end
end)

Thanks

Thanks for reading, hope I helped.

:-)