Thanks for all the interest, everyone. I'm a bit blown away by how considered all the comments have been.
To answer some of the questions:
I ran the Manchester marathon in 2013 after a build up interrupted by injury (accidental rather than stress injury). I didn't have the miles in my legs and really struggled in the last 10k, finishing in 3:57.
Since then I've joined a club and now run four times a week, hacking large chunks off all my PBs. I ran the Coniston trail marathon back in June, which was very hard and took me about 4:20. The last 20 minutes were a struggle.
All year I've been running 18-20 miles about once a month, just for the hell of it. Now I'm building for the Robin Hood Nottingham marathon. I did a 19 mile run on Sunday, which was mostly hard trail with the last 6km run on road at 5min/km. So I think I'm looking pretty good for the 3:30 target.
I have a HM booked for 2 weeks before my marathon. (I know that's a bit close for some but, well, I've had to move the mara due to a family wedding and I can't afford to cancel all these other races). I'm not going to kill myself on that, but whatever I do wrt the effort level may inform my marathon target.
One more consideration: I'm 48 years old, so I'm not taking for granted that my knees will allow too many goes at this.
Following the advice above and some deep and profound meditation, my plan is at the moment is going to be...
a) Set out with a firm target of 3:30. Start at very strict even pacing of 5min/km. Hold my fire until 30km and then, if it's looking good, start upping the ante for a negative split and a thirty minute PB.
(Best laid plans an all that. I'll report back on results)
My PB for a half is 1:37, but that was a year ago. I think I would target about 1:32 for a half now.
I've run 18-20 miles quite a few times this year. I ran a hilly trail marathon back in June and ran myself out with about 5km to go. I was quite happy with 4:20 as it was a tough course (I didn't mention this in the OP as it muddies the water a bit). Most of my long runs have also been on trails, but I very rarely get more than 40mpw in.
I agree with you, piscator. My limited experience is that the mara is so specialised that even a HM time is barely worthwhile as an indicator. I think I've already decided to go for option a) and the potential joy of a strong finish.
[This may look like a humblebrag, but it's a genuine dilemna]
I'm building up to my second marathon, having done my first in 3:58 a couple of years ago. Since then, all my other running has improved loads. I've been training with a target finish of 3:30 and have done a 25km training run at race pace of 5min/km.
I'm racing 10k in about 40mins these days and all the predictors seem to suggest I should be aiming higher for the marathon, but I really don't want to go out too fast and blow it altogether.
What do you think? Should I
a) stick to targetting 3:30 - it'd be a 30min PB and maybe I'd breeze over the line with a negative split, or
b) change my target to 3:15 - the numbers don't lie and for all I know this might be the best shot I'll ever get
Obviously this is a binding vote and I will abide by the decision of the crowd
At the moment I'm smashing everything. I looks like it might be a great year. My sping schedule is packed, culminating in a trail marathon in June. I reckon I might be up for a shot at a fast flat marathon in autumn, but here's the thing:
Having signed up to a few races in the past and pulled out for one reason or another, I'm wary of comitting to an autumn marathon now. Are there any nice flat marathons that'll still have any entries open in, say, July? Or will I end up scrabbling around for transfers if I don't sign up now?