Your story sounds quite similar to mine as I ran my 1st marathon in 2012 (aged 47) with the goal of a sub 3 (had ran sub 1:25 for the half previously). I had a dissapointing run on the day (Milton Keynes 2012) clocking 3:13:38 and felt pretty much the same as you do now.
However I consoled myself with the fact I'd achieved a GFA time (sub 3:15) for London 2013 & then spent a year training for it culminating in completing the previously mentioned 18 week P & D 55-70 mpw plan. I subsequently ran 2:59:05 last April to achieve the sub 3 goal - needless to say I felt great!
I gave it a miss this year but am planning on using my still valid FGFA time from last year to enter next years race with (hopefully) a similar result. I stuck religiously to the P & D plan running every single session at the prescribed efforts over the 18 weeks & would highly recommended it.
I'd urge you not to give up as it definately sounds as though you can go sub 3 - good luck!
I ran the VLM in 2013 clocking 02:59:05 (aged 48) - I want to run the race next year (gave it a miss this time round) , am I correct in assuming that my time from last year will give me automatic entry in 2015 (I hope so!)
Would be grateful if someone could confirm this for me