So you never revisited the same areas? Here's the route that you would take if you want to finish the game with the proper ending:
Cordon » Garbage » Agroprom » Agroprom Underground » Agroprom » Garbage » Bar » Garbage » Dark Valley » X18 » Dark Valley » Cordon » Garbage » Bar » Rostok » Yantar » X16 » Yantar » Rostok » Bar » Garbage » Cordon » Garbage » Agroprom » Agroprom » Underground » Agroprom » Garbage » Bar » Army Warehouses » Red Forest » X10 » Red Forest » Pripyat » Chernobyl Exterior » Chernobyl Interior » Portal Stage *
Notice that, for example, if you finished the game, you visited Garbage 7 times at least. Also, once you reach the Bar, you can go to the Army Warehouses, do some quests for Freedom, then return, go to Rostok (Wild Territory), scrounge for artifacts and do some quests, etc. There's no ticking clock that's forcing you to go immediately to the next goal for the main quest line. I challenge you to name an FPS that is less linear than S.T.A.L.K.E.R. Shadow of Chernobyl.
* I have this stuff on hand because I used to speedrun this game using the brilliant Redux mod (without using game glitches) with an English buddy. My best time, start to finish, 4h24m. There are parts in the sequence that you can skip and still complete the game because it's not linear.