I was going to be all sanctimonious and comment on who on earth fancies a burger after doing a marathon until I realised that after Frankfurt, there were 3 forumites who ate at McDonalds before hitting the bar!
You certainly have the basic speed David and if your aim is to beat the 3hour mark, sensible pacing should help you achieve that relatively comfortably. As you gain experience you are likely to go well under 3 hours.
I've been researching over on the USA Runnersworld.com site and there is heaps of useful information over on the forums there. I don't want to kill this thread but if you like to be prepared and have questions you are too shy to ask then pop over the pond and read away. There's even a thread on the best burger joint for after the marathon!
Wenty what is the idea doing Barcelona so close to London? Are you aiming for sub 3 at Barcelona, London or both? In my opinion London is a much faster course but I would be worried that the two runs were a little close. Both great fun to run though.