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.

:-)