I'm a tad younger (almost 40) and off a 1:46 GNR last year managed a 4:18 first marathon in Chester (off 16 weeks total training and 2 stone overweight inc the GNR three weeks earlier than the marathon). I was aiming sub 3:50 (overly ambitious I was told for a 1st marathon, but I didn't listen!) and on course, until knee issues due to fatigue at miles 18-20 onward (walk, limp run rotation every 50m). I'm fairly sure I could have done sub 4:00 with an extra 2 months of prep and a more long runs (or maybe even just setting off slower) - long runs... I think this is the key to get three or four 20+ milers in beforehand, I only managed two 18-19 milers.
As you can see above - I'd imagine this is a typical collapse, slowing from a too quick start - 8:30 min/miles up to 16.5 miles to too slow finish - 12 min/miles as fatigue and poor form induced knee pain.
If you kept your training ticking over since the GNR and you pace yourself sensibly you should be on course for 4:00-4:20 I'd have thought. Then again if it's a hot day in Barcelona that could increase a fair but!
I was gutted with 4:18 for a couple of hours after the finish - but in hindsight and reading others on these forums, now I'm rather pleased with it for a first marathon because *I finished*! Just enjoy it regardless of time and plan for a good time in your "second" marathon.
Edited: 04/01/2013 at 13:01