If you are in it for a fast time, don't do London. Bloody simple really! It's obvious that 36000 runners are going to cause congestion so why are you even trying for a time?
I am one of those people who cannot run on an empty stomach but obviously not on a full one either. Depending on how hungry I am and how far I want to go I will toast a bagel or two before a run and set off within half an hour or so. And I recently discovered that if I have an energy gel (experimenting before the marathon - see I do read the manuals!) I tend to run pretty well. Well, better than my usual slow plod.
I do not drink (well, very rarely these days - about once every few months I have a night out and give it a good bash).
I do a speed session once a week (buitl up gradually but it's now either 10X400m or 6X600m with 200m walk recoveries between) but this doesn't seem to make any difference to my LSR pace.
Despite living in a seaside town, I do not swim. Can't. And some bugger 'borrowed' my bike a few months ago and hasn't brought it back.