One of the hardest things about writing a novel is to keep at it, even when it's driving you nuts. I recently had the problem of my story spiraling out of control. I was going at a good pace, was writing huge amounts everyday and was getting closer and closer to the finish. Only to realize I had really overestimated how much I was putting into each scene and how long each scene would be (or how short since they were mostly too long). Worse, I realized that for the romance genre the novel I was writing was going to end up waaaaay too huge for what it was.
Now the problem I ran into was that the things I wanted to do were not supported by the earlier story AND I had put too many characters in. This is fixed easily enough by taking the characters out and by removing those scenes that are basically repetitive but I also realized I had not taken the time to build enough into the story. Now with this "rewrite" which is basically me going through and adding scenes from the male protagonist's point of view to balance the female protagonist's point of view and taking out the other character's POVs that really didn't need to be in there I'm feeling much better about the novel as a whole, but man is it tiring.
Don't ever let anyone tell you writing a novel is easy. Yes, you're sitting there and you're typing and physically it might not be very active or very 'hard' in that sense but you do use a lot of your energy and your mental faculties trying to remember every little detail you need. You also have to really focus your mind so that you can get done the amount you want to get done. Add to that the ache that you can get in your hands (hah, and you thought there was NO physicality to this) while you're typing because you don't have exactly the right setup for your keyboard to relieve hand strain as much as possible in typing. If you're like me and you're a very particular temperature person (I need my hands to be a particular temperature, if they get too cold my typing speed drops significantly to the point where I can no longer continue typing because I can't feel where the keys are.) then the location you choose is desperately important. You need to know how it's going to fluctuate in temperature and if you can control it or not. I'm writing today at a Starbucks and they love, LOVE keeping the air conditioning on despite the wickedly cold winter days that are happening outside. Fortunately I've thought to dress warmly and to sit with my computer in my lap. Strangely enough this is helping keep my hands warm enough to type comfortably.
The music and the people make it more difficult for me to concentrate as I'm outside of the quiet of my house but it's a trade off since I don't have the easy access to games that I usually have at home. The sound is a lot easier to combat too since I found Songza and realized that I could pump classical into the back of my head to block out most of the intrusions and to help me focus. The music is not always ideal for my writing since I sometimes find it distracting, but Songza certainly does have some wonderful playlists to help with that and I've found it a valuable tool in the writing process. It takes me a bit to get everything set up the way I want it, but once it's there whooo man I can write a lot.
Find your groove, pay attention to what you're doing and to how well you can write in any given situation. Notice what your distractions are and try to minimize them, then focus, sit down and write. It's the only way you're ever going to get it done, even if that means rewriting a lot earlier than you expected. Don't worry, you can do it. Ironically I am also coming to understand that a wonderful way to increase your stamina (mental and otherwise) is to keep yourself at a high enough fitness level that the marathons of writing don't actually harm you too badly. A healthy body really is a healthy mind and I do suggest at least trying to keep both in balance, it does seem to help with the writing part.
All in all the writing is slow, but is moving steadily and ideally I'll be able to figure out how to make them move more than one day a week at this pace. Find your own rhythm, everyone's is different, and play to that. By paying attention to things that help you focus you'll at least stand a chance of being able to reach the optimal focusing zone.
Good luck and God Bless!