Regarding pacing - Running a negative split is desirable, though in practice may be unlikely! Running as even paced as possible is more likely and even though you may slow down a bit, and the vast majority of people do, it shouldn't affect your finish time too much. If you are aiming for 4 hours, I would agree that running at 3.55 -3.58 pace could be the thing to. It gives a bit of time 'in the bank' but isn't wildly over ambitious.
Taking on fluids is vital, and especially early on even if you don't feel particularly thirsty or tired in the early stages. I would reccomend trying to drink at least 200ml of the carbo drink on the course at every station as well as water where possible. Also use the water sponges and showers to cool you down. It will all help later on.
Enjoy the first 16 miles but expect it to get hard in the last 10 and thats where your training comes in.
All the very very best of luck