Personally I don't think any of us really had a disastrous day.
We all finished and that's the most important thing. Sure there are disappointments where we didn't run as fast as we should have, got injured or felt bad, spent 5 years of entering the ballot to do nothing spectacular at the end of it etc but as said, we crossed that line along with 1000s of others of all abilities.
I personally never really think about 20 miles + 6.2 miles.
I tend to think of "once I get past 16.2 miles, mileage wise, it's all single figures from there on which is pretty much a medium length training run".
Long run wise, haven't done any in a couple of years. Used to run 18-22 milers every weekend but then dropped back to maybe 4-5 x 8-10 milers a week (35-45 miles generally with no tapering) and started to improve (although I'm inconsistent but mainly state of mind). Before my last marathon at Loch Ness, I'd run Moray 4 weeks before and got a 3h35:30 PB there and it was fairly comfortable. 3 days before Loch Ness, I went out for a run in the evening and ran 10 miles in sub-1h09 which was one of my fastest times. Then an easy hilly marathon - could have been 5 mins or so faster but stopped for the toilet at 23.xx miles (should have held on as I wasn't that desperate). Ran that on a mouthful or two of an Innocent smoothie at 5:50am, "half a Go Ahead bar before the race (9:50am) and a couple of mouthfuls of water after my toilet stop)
Pace makers do help me a lot to be honest. During Loch Ness, I tailed a girl for a large part of the race who was running at my comfort pace. Moray had me chatting to some people whilst running along at a good pace as well.
London is too variable what with people weaving in and out of outs, shoving you aside, constantly hitting people doing 12min/mile pace etc. I find that difficult to run in as I can't get a steady pace sorted out and constantly changing doesn't help.
I finished so I suppose that's all that really matters.
Ran my last 2 marathons back in September 2012, both in around 3h35 (PBs in both).
Got injured after changing shoes to some Nimbus 14s and never really managed that much in the way of training after that so a good 4-5 months of doing very little. Managed a few "long" runs of just over 8 miles / 1hr and a handful of 5-6 milers, with peak weekly mileages of 27 miles (twice) about 3-4 weeks ago,
Started off slowly and hit the halfway point at 1h57, some 20 minutes slower than in training - not easy finding space to get any sort of pace so just plodded along with the flow. Took water on for the first time at 14 miles or so and after that it was pretty much walk/run with too much walking that decimated my time. Seemed to lose a lot of salt and legs were cramping a bit although to be honest I should have pushed through it more - quite capable so too much of a defeatist attitude from me rather than lack of ability even after little training.
Second worst marathon time for me (nearly every dodgy time I've had has been in April followed by May!)