Rob22: Negative splitting is very hard to do, your effort can be harder in the 2nd half but the pace may be the same. To practice this running longer sessions specific to marathon pace and just a little faster, and also doing some faster paced running towards the 2nd half of your long runs, so you practice increasing pace when you are tired.
Inlastplace: Drinking pre marathon should really start the day before, making sure your are checking your urine colour and that it is pale to clear. Sip little and often, having a drink by the bed, and then when you wake just sipping on an electrolyte drink, up to about an hour before the race, when you may back off the drinking, before you start. Drinking about 500ml should be plenty if you are hydrated previously.