Both the solutions here are correct, but I wanted to add that you can easily 100% the game well within the time limit if you only want to go through one playthrough. I played on Uber difficulty, got 100% of the collectibles and side quests, got every other single player achievement, and still beat the game in under 9 hours and I could have probably shaved another hour off of that if I was really going for a speed run. I also didn't skip any cut scenes, although I'm not sure if those count towards the time or not. I did rush through some of the dialog which I regret doing since I ended up having so much extra time. My recommendation would be to not rush through dialog (i.e. moving away from people while they are talking to you when they have something interesting to say) just to save a little time.
My recommendations to keep the time down plus various tips that might help overall that I wish I had know before starting are:
-Don't collect any of the Midtown East/West collectibles until after the farm mission, unless you have a map that you check off each individual collectible each time you get it (I found a very good place for this is at http://www.supercheats.com/guides/wolfenstein/collectibles
). Part of the city is blocked off until after the hospital mission becomes available, so you cannot 100% the city until then. Same goes for Downtown collectibles until after the Castle mission is over and the Airfield mission becomes available. You can't 100% Downtown until then. I say this because going through and collecting some of the collectibles without marking them off on a map is very likely to cause problems in the end when you decide to finish off the area. You will end up spending more time re-visiting places you already went to and possibly looking around for a collectible you've already gotten, wondering if you just can't find it, than you would if you marked it down on the map each time you get one (i.e. copying the map at the aforementioned website, pasting it into MS Paint, and then putting a colored dot over each collectible once you get it). I would recommend this method over saving it all to do at once if you are trying to save the most time, as you find yourself purposely ignoring collectibles and having to go back to various corners of town if you save it all to do it at once, where you will save a lot of time if you collect the things the first time you visit the area.
-Make sure you 100% all levels and towns before starting the Airfield mission. There's no going back once you start this.
-The counter the game goes by is the in game time played counter in the menu. When paused, the counter doesn't go up, so make sure you pause while referencing a guide.
-The game tells you what % complete you are with the game, which includes all the collectibles. Thus, you can use that as a vague guide to tell you if you are behind, on pace, or (more likely than not) ahead of pace to beat the game in time. For instance if it says you are 50% done with the game, but you've only played for 5 hours, you are on an approximate pace to beat the game in 10 hours.
-You are going to want to get all the collectibles on a level the first time you play it, so it is best to reference a guide. There are some great video guides on YouTube that I followed to 100% each level and they worked perfectly.
-One of the Midtown collectibles is only obtainable directly after finishing the the Paranormal Base mission. If you miss it the first time, you can just select to replay the mission from the mission selection menu and immediately abort it, and you will end up where you need to be.
-Don't stress too much about what weapon/veil upgrades to buy. Once you collect all tomes, all the veil upgrades are automatically unlocked, and once you collect all intel all the weapons upgrades are unlocked. This can help a lot on the final boss.
Finally, I found myself kind of stressed to save time the entire game and wasn't really enjoying the game as much as a result. If you find yourself feeling the same way, even with my reassurances that you will have plenty of time, you might just want to do two separate playthroughs. Good luck!