HCE is still one of the most epic campaigns. What was it - 15 hrs? It took a little while.
Umm... No.
If we are on the same page and HCE = Halo: Combat Evolved... Or just Halo. 15 hours it most certainly was not. There was a whole club that was focused on people who could beat it on all the difficulty settings solo in less then 24 hours. That means beating the whole campaign on easy, normal, heroic, and legendary in 24 hours. I did it in 23hr 28min.
Halo 3 was by far the longest single player campaign so far, and on release night some friends and I powered through 4 player legendary in just over 6 hours. While playing by yourself definitely extends it, it still isn't very long.
The Halo games have always been short, however most people play through multiple times on the different settings, or looking for skulls, and so on. Then of course it is a HIGHLY multiplayer orieneted game.