Re: OOC.
You are indeed in a 10 ft wide, 9 ft. tall tunnel, the creature is at present 40 ft. in front of you. You have not yet been in, or even seen where the tunnel opens up. You can still only see tunnel in front of you, the twisting walls preventing you from seeing further. The marching order is correct.
A lantern would not be light, its an improvised weapon, dealing 1d4 bludgeoning damage, though if used as such it might break, possibly doing fire damage to the wielder and target.