I finished COD4 in about 7 hours on Normal. I skipped Hard and am playing Veteran ... insane and will get worse as there were two spots that were extremely difficult on normal; you'll find out. I like the game and I also enjoy it online playing against others. There are so many different game modes to play such as old school, team death matches, free for all, sabotage, search and destroy, headquarters, domination ... and so much more.
If you are good at First Person Shooters (FPS) Cord, I would suggest starting on Hard to get challenged. I think you will like the game and equipment you will get to use. Modern weapons and armament is cool.
"IF I COME ... I'M BRINGING THE PAIN WITH ME"