Just to throw my lot in here...
I ran the London Marathon last weekend - it was a pretty incredible experience. Never run a marathon before but you can tell London is something special, the crowds are insane (to the point where you almost start to appreciate the quiet moments more).
Started running relatively late last year without any intention of doing a marathon but the chance came up to run London and it felt like too good an opportunity to turn down. Was about 98kg in July and I'm now floating around 90 (the fitness may have increased but the diet unfortunately isn't any better).
Went out with a goal of 3.5 hours but realised about 60% of the way through that wasn't likely to be possible (so much weaving throughout you almost end up running a full extra kilometre due to all the lateral movement). Readjusted my goal and ended up walking for about 500 metres until some jellybabes from a crowd member gave me the impetus needed to run the final 5k (I fast on long run days because my stomach is unreliable at the best of times so I was tanking energy-wise). Finished up just under 3:45 which I was reasonably happy with.
While the run was tough, it's the prolonged training that's tougher - if you can get through that then you can definitely get through the marathon on the day! Good luck to everyone running one in the coming months - it's an awesome achievement.