Accidentally beat the game in 3 hours

My mate and I made a new world and after looting the plane we went in a cave to get some loot, we got the rebreather (and keycard) which we both agreed was lame so we went into another trying to get a cool item. Turned out to be the big cave and we just played through, beat the boss and chose to save Timmy. Neither of us had ever beaten the game, thought it was odd you could do it so quick.