Okay, have to ask, when did 13 hours become short for an FPS? RPGs are another kettle of fish entirely (and that includes Borderlands), so the trade-off for length in games like the Witcher, Borderlands, Mass Effect, etc., are usually lack of multiplayer and combat that isn't going to match an FPS. Not saying that the combat in those games is bad, but, well, I've been playing Xenoblade Chronicles for the past year, I've got a playtime of around 80 hours, and thanks to doing various sidequests, I'm not even up to Egil yet (in Agniratha though). Is it a good game? Yes. But that length has to factor in travel time, down time, and a combat system that, while not bad, isn't really the main focus of the game.
So, using other examples - I'd say Halo 5 took me about 6-8 hours in its campaign, plus around four hours spent on multiplayer. The original Doom games I'd say took around a similar period of time in the campaign. FPS games are usually on constant burn, so to speak, so 13 hours sounds fine. Honestly, playing through the early Doom 1, I started feeling burnt out once I reached Hell - lack of any new enemies, lack of any character or story (not that I expected it, but by this point in the game it had become a downer for me), and then, thanks to the BFG edition, I played a selection of extra levels past the mastermind. Apart from a sandbox FPS like Far Cry, I don't think any FPS could have a campaign the length of an RPG without giving the player burnout along the way. I mean, there was GoldenEye that kept me occupied for ages, but now, I could probably beat it in a similar timeframe to the figures listed above.