Well was it your first playthrough ?

Veteran was obviously my second, so I skipped the loading scenes and already knew what to do. There were also hardly any parts that were really difficult. One in Vorkuta I think (before you get the mini gun) and one very short part at the end of the rooftop level (which I found incredibly bad designed on Veteran. Only talking about this last scene with the pistol, but also the whole level was probably the hardest. Took me an hour alone to finish it.) but nothing else AFAIR. 

Really? I thought that it would be the 3rd last level, where I was convinced I had infinite enemies running at me

Which one ? Rebirth ?

yes I can confirm that there are parts with infinite spawning enemies. ( also in some other levels)

There was this long hallway that was filled with enemies and it was pretty hard to advance, right ?

Nevertheless the rest of the level wasn't that hard IMO.

I was thinking in the labs, very near the end

In the labs....

I really don't know if the place I mentioned and the one you mentioned is the same level.

I know for sure yours is Rebirth. You're talking about the Mason/Reznov part of the level right ?
I remember the scene were you come into a room and someone throws  a desk on the floor right in front of you and behind that is the hallway I'm talking about. Should be a bit after you first enter the labs, which were a bit annoying (died a couple of times on Regular because I was too impatient).

You also had a crossbow as a secondary weapon.

Guess it is good I didnt buy it for the online games because I went to try it and sat waiting for 10mins with it saying please wait.

The games has glitches.  You need to make a second profile and log in online with it (no need to join a match) then log back off to get your first profile to start working.

Without the multiplayer, this game isn't worth it.  I wish they'd go back to CoD 1 style of single player.  The story was well told without interupting the flow of the game.