Half Life is well over a Decade old, unless you played it in it's time (when it won 5 game of the year awards in a row) then you'll never fully appreciate its true quality because it's 14 years out of context. It was one of the most immersive games i've ever played and my god its length. It'll take you a whole week if not more to complete, you deffinately get your moneys worth.
Apparently 2 player Conviction is a different campaign altogether? You play as the trainee's in training, who will be after the main character in the main game. I wouldn't be surprised if it was shorter.