Below was adapted from a recent post on RW - just some of my thoughts after a friend had to drop out of a marathon before halfway despite a strong build up with multiple consecutive 100 mile weeks... Be good to get all your thoughts.
A tuppence from someone incredibly inexperienced in running marathons but who has run a lot of races; my gut feeling is that the first half is all in your head and the second half is about your heart.
When you know you've done the training and you know you're in shape and you're struggling and off the pace within the first half then something is not right. Either you've gone off at the wrong pace, you've got an underlying injury issue or an underlying virus.
If you're running for a team (or representing your nation) then you do not drop out if at all possible. If you're running for yourself, and you want to perform to your best, then what use is continuing on for over 13 miles? What are you trying to prove? That you can keep your legs moving for 13 more miles? Big deal - so can 30,000 at London - and most of them are wearing bunny suits. I think most reading this blog are looking for more than that, they're looking for a special performance to hit their absolute limit and if you're not going well at 12 miles then that's not going to happen.
I've got absolutely no doubts as to my mental toughness when I'm on the start line. I've hardened myself through gut-wrenching work-outs, lonely sojourns at marathon pace in the pitch black and the general debilitation and grind of running mileage week in, week out the same as we all have. It's when you're running before the sun is up and you're running in the twilight as the sun sets. So if my body is telling me at mile 10,11,12 - that I'm already badly off the pace and it's not going to be happening I'll be straight off the course, into an icebath and within a month I'll be ready to put my body on the line again and this time, it'll go to plan because I've done the prep and if it doesn't then I need to go back to the drawing board.
Way I see it - if you carry on when something is clearly not right you're trying to prove to yourself or to others that you're mentally tough enough. If you've got the self-confidence to know you are then it really doesn't matter, you can just focus on hitting your target.
Different in shorter races, but the marathon is a different beast and needs to be respected. In the second half you need to dig deep and find everything you've got; the first half you need to be cool, calm, collected and rational.
As a side note - I have only ever dropped out of two races.
The first was my second ever half marathon the day after I ended up stranded moving continually at about 0.5 miles per hour for 5 hours (they were letting a 2.5 mile tailback off the motorway with traffic lights...) - I missed the national XC and my back was going into spasms when I got home, found a race the next morning, set off very well, but about 4-5 miles my back started tightening, mile 6-8 were in agony and at mile 8 I just had to lie on the pavement for about 10 minutes until I could get up again.
The second was at BUCS XC in Cardiff. I shouldn't have been on the start line. My calf had been in agony for days but intensive massage the previous day after the long minibus ride had seemed to have eased it off. Unfortunately it was still not great but I decided to run anyway, ironically it was absolutely fine and my back again totally screwed me over and left me hunched in agony.
Generally I've discovered that minibuses totally screw my back-up and I need to be majorly stretching at every single stop or else staying over the night before to race well. I think this is probably part of the reason (together with my race usually being the last one and hence the course absolutely torn up) that in previous years I have run relatively worse in the major XCs than in smaller ones.