Good luck. I did it in 2014 and saw footy in the evening (Barcelona won 7-0!). Really well organised event and as other posters say expo, start and finish all in the same place so logistics are really easy.
The race didn't go well for me - it got really hot and I don't work well in heat (I know - what was I expecting).
1 other point - you have to be so careful with pickpockets in Barcelona - they work in gangs and are very organised. Have one of those moneybelt things
Hi BillyBudd - April is a long time away yet and I would say its too early to set a target time, just do the training and then see how that goes and then set yourself a range of target times
1) In the bag, could do this time with my eyes closed
2) Realistic, I'd be happy with this
3) Dream time - this shouldn't be 2:30 or something, but say if everything goes really well on the day, so perhaps double your half time (from a half you do in Spring or Gosport) + 5 - 10 minutes
Don't skimp the long runs, I've done 2 marathons before and the pain you feel at 20 miles is the toughest thing - get through that and you are done.
I also did GSR on Sunday (72:25) and am pondering on Gosport, currently trying to build up my long runs by 1 mile a week, so GSR at 10m last week, this week, 11m, next week 12m, Gosport half fits quite nicely into that sequence. When I get to 20 + (around Christmas) I'm going to try and keep that going into the Spring as that is the point which finished me in Vienna and Barcelona so I need more long runs I think