I carry my own water only because I haven't mastered the art of drinking out of a small cup while on the run. If you're someone that takes walk breaks, you can use the water stations as a walk break. In that case, I wouldn't carry any water. As far as fuel, use whatever you have been training with. Do not try anything new on race day. Stick with what you have been doing. Some people don't need anything, others do. I've tried a wide variety of fuels & the only thing that hasn't messed with my stomach is pure honey. I pour a "mouthful" in a baggie & carry it with me. I fuel at miles 12, 17, and 22. Some people do earlier. I tried it at 7 & ended up with cramping by 13. It's all trial & error & what works best for YOU. Good luck. I'm envious of your London marathon!

I did not carry water, shouldn't be necessary with a well supported race. Pinch the top of the paper cup so it doesn't slosh all over and then sip it out of the side. I used Gu for fuel, I like the chocolate flavours. Use whatever fuel you used during training. I got a SPI belt with the little loops so I could easily grab them. I also carried some kleenex (tissue) because my nose runs while running I am not capable of performing a snot rocket. :laugh: But there were actually people along the course handing out kleenex. As well as vaseline, oranges, pretzels, beer, cookies, apples, and I forget what all else. You really don't need to take much and you're not going to want to be lugging all kinds of stuff for 26.2 miles.
As for how much fuel to take, that will depend on how fast you run and on what has worked for you on your long training runs. Look at your course map and see where the aid stations are so you can plan. Take your gels with water, not Gatorade. I believe the "rule" is 60 grams of carbs per hour. I took a Gu every 5 miles, including one before I started. I I then took Gatorade in between. I actually wrote on my arm what miles the stops were at and what I was taking at each one so I wouldn't forget or get confused.

Absolutely no need to pack water unless it is an unusually hot day. I just looked at the course map and they have water stops starting at mile 2, and then nearly every mile thereafter. You will have plenty of opportunities to drink! If it is super hot, you might consider wearing a hydration pack because big races are known to run out of water if the conditions become unexpectedly hot. But if it is normal weather there is no need to carry at all!

At this point you should be figuring out the types of calories you can handle consuming during the race -- Gels are the easiest to eat and carry, and typically you would consume one every 45 minutes. Some people prefer sports drinks, so find out what drinks are provided by the event and try that out to see if you like it (It looks like they gave out Lucozade last year, but I couldn't find out the info for this year).

Otherwise, just make sure you have layers of clothing that you can discard as the temperature warms up. You will be doing a lot of sitting around before the start and you want to stay warm! I like to go to a used clothing store before the race and buy a cheap set of sweats. That way I can just toss them at an aid station where they will be picked up and donated to charity.

Good luck!

All good advice above! I can't really add anything because I won't have my first marathon for another 8 months.

But I thought I would share this neat calculator my wife found. It's about "hitting the wall" and refueling. I don't know how accurate it really is but it's interesting.

https://dl.dropboxusercontent.com/u/5922669/Marathon Wall Calculator.xls

Absolutely have nailed down what you will wear on race day down to your underpants. Anything that would be a small irritation on a regular day will become a major pain after 26.2 miles. Unless you can find info about exactly what sports drink or fuel the race is provided and train with that I would say bring your own. Water specifically I think has been addressed for and against so you'll decide what makes you more comfortable. It's been said and always bears repeating... NOTHING NEW ON RACE DAY! Good luck! Enjoy it!

I haven't done a marathon yet, but I learned from my HM to carry a few bandaids or pre-cut mole skin just incase you feel a blister starting. And if you can get the body glide samples (the little tiny bottles), stock up on them and put one or two in your race belt incase you need to apply more mid race.