Every MGS game since MGS2 has had a prologue, they all last 2 hours-ish. For the main story mission, this is no different. But combine the extra ops, unlockables and hidden things, you'll get 15+ easily.
Great gameplay. There are SO many different ways to complete the tasks. I stealthed on foot, mostly on roof tops without being spotted while my friend hitched a ride on a truck thrn stole said truck to escape back. Truly innovative stealth gameplay and ridiculously satisfying.