It's more than fairly easy - especially after you go back and try it on Normal after playing on Heroic or Legendary.  It's a joke.  One step below (Easy) that and the description by Bungie is "This game basically plays itself".

I've beaten the campaign on Heroic and don't even want to try Legendary because I'm afraid it would take too long.

However, unless I was playing co-op with a friend or skull hunting, I wouldn't want to play past the 2 levels I've re-done on Normal because it's insanely easy. I died 2 times on each level and that's only because I was running forward seeing how crazy I could get with the Melee kills.

A buddy of mine beat it in 2 player Co-op (on normal I think) in ~5-6 hours.  He's played Halo 1 and Halo 2, but he's not "awesome" at either.

This is only heresay as I didn't see him do it and I'm pretty sure he didn't use a stop watch to time himself, but we're not looking at a super long game however you split it.

One thing that people don't realize is that Bungie nerfed "normal" and they recommend that anyone with previous Halo experience step up to a Heroic or Legendary, two difficulty levels they bumped up to compensate for normal.

After playing through on coop Legendary and most of single player Heroic, I can definitely say they increased the difficulty.

I also played around on Normal for awhile and it was comical. I beat the first two levels without stopping once to even reload (shit, on the first level, I meleed the entire group of grunts and brutes in that big open area near the cliffs).

The only level I had to play on co-op legendary to get past was Cortana - and thats because the turrets are fucking awful

That doesn't make the actual game longer though.

You don't suddenly have a bigger game because you die and get to redo a portion of it.

Easy or hard, game length is game length.