Its honestly worth the effort. The more the information you give them now and the better and more detailed the 'picture' you give them now about your daily life and limitations in writing, the less the F2F will actually contribute. If you can give them all the information in the level of detail they need to understand what your daily life is like, the details of how you are affected, how you actually live day to day and what your medical issues actually are, then the less point there is in the whole F2F process.
At the F2F after a few niceties, essentially all they do is go through and ask you exactly the same set of questions as are on the form. They type your answers into an online version of the form as they go along which ultimately is used to generate their 'report'. I had invested a lot of time and effort in providing a lot of detail about my medical conditions and how they affect me with examples and it meant that the F2F was pretty much a waste of everyone's time. I had written my answers in exactly the same way as I would describe my situation if I had been asked the questions by someone so the assessor was able to skip over all of them knowing that he didn't need to write much down because everything he needed was in my form already. The gallop through the questions was really just cursory because he had to do it - the information he was looking for was already there and he knew that. I don't think he even had to use the F2F to clarify anything I'd provided in my written answers.
I wrote nothing at all in the boxes on the form - I just ticked the 'tick-boxes' and put my whole written answer to each question in a single appendix, numbering each answer with the relevant question number and question title from the form. I reckon that my written answers ran to about 6 or 7 pages of printed A4 in a fairly small font. Its counterproductive to provide far too much information - it makes it impossible for them to find the salient information they really need but if you give too little then you give them nothing to go on and the F2F starts to become much more critical. Its always easier to get more information down on paper when you have time to think about it, aren't under direct pressure and can keep going back to add or re-word things. In a face to face situation where you are asked verbal questions its inevitable that you have only a fraction of the time to give your answers, you have to think on the spot and formulate your answers 'there and then', and you don't have the luxury of being able to go back to change, re-word or add things afterwards. I was also in the very unusual situation of having had a DWP appointed doctor come out to my house to do an assessment for my previous award of DLA and I had had a really good outcome so I specifically asked them to take that DLA claim into account as part of the information they should consider when they were assessing my PIP application. They also had 4 completed ESA assessments/re-assessments which they clearly used as well.
I'm sure you are doing a great job and it will all be worth it when you are finished. Don't forget to keep a copy before you send the form off so that you have it to hand to refer to in the F2F and for any future re-assessments. It makes life 100x easier!!!