I've read that they reckon that it takes at least 5 or 6 marathons to get a feel for how to run one, pace yourself, get prepared properly etc etc.
I ran FLM in 2007 and the heat killed me, but also the travelling down, collecting number, trekking across London, catching trains, little sleep the night before .... so stressful and so tiring (before even running the race!). Since then, I've pb'd at Windermere and beaten my London time at Snowdonia on two occasions!
So, I wouldn't be too hard on yourself, put this one down to experience, definitely consider running a smaller marathon with fewer logistical problems (I've heard the Wales Marathon is brill!) and have another go!
Remember, it's not just the London marathon that's 26.2 miles 
PS Can definitely recommend Snowdonia - little chance of overheating there!!
Edited: 20/04/2011 at 22:20