I completed the game 100% (easy, medium, hard and black ops) in 19hrs 45mins. For levels that, according to one guide should take over an hour, only 2 levels throughout the entire game took me over an hour, the last level on black ops and on hard.
To a certain extent you can just run and gun and blast everything to hell with plenty of health packs on easy but, even on "easy" the later levels were fairly challenging. Not to mention I'm still trying to beat the last level. Excellent game. Maybe I'm just getting old and losing my touch, I don't know but the easy level on this game is just enough of a challenge to make this a great game.