Sub 4 hour marathon - target or pipe dream?

I've been aiming for a sub 4:15 marathon for over a year now. I missed out at London Marathon 2012, with an angry ITB foiling my first attempt. I missed out at New York 2012, due to the race being cancelled. And this year I missed out at London again, by a mere 76 seconds.

I've spent the months since April wondering if I can really improve any more. I'm 40 so I'm not really sure how much improvement I have left. Some reading I've done seems to think I can only hope to maintain my pace, other opinions are that as I haven't been running that long (7 years) I've got a bit more scope for improvement.

On the positive side, since I turned 40, I've PBed twice at 10k and half marathon distance, as well as notching up 5k and 10 mile PBs too. So I like to think that I've actually benefitted from turning 40, rather than gone over the hill.

My next marathon is York on October 20. I'm always more confident about autumn marathons - I like summer training and hate winter training, so always feel better prepared for an autumn race. In the first couple of weeks of training, I had a couple of (accidental) strongly paced runs which indicated that I might be able to run a bit faster than 4:15. I started to wonder, should I aim high and train for a sub 4 hour target?

So I've subtly been doing this. Not publicising it to anyone much, just monitoring my performance and seeing how it maps across to a 9:09 race pace. My inaugural race at 5 mile distance in the anniversary run in July indicated that my PB for a 10k could be reduced by 3-4 minutes again at current form. But I'm experienced enough to know that short distance improvements don't always translate to long distance pace for me.

Last Saturday, I tackled my first 'long run', which for me is anything over 13 miles in marathon training. 16 miles, a long slow run. I know can run a half marathon comfortably below 9:09, (my PB is 1:57:15) so this was my first real test to see if I could maintain a faster pace (for me) over a longer distance.

Miles 1 and 2 were comfortable about 9:39 pace. Maybe even a little too fast for what is supposed to be a long slow run. But I thought I'd keep going and see how long I could maintain it for. Miles 3, 4, 5, 6 and 7 - all at a similar pace. Mile 8 saw me drop to 10:08 but this was running up Richmond Hill so I don't feel too bad about this slip. I managed to run 15.5 miles in the end, in just under 2.5 hours, averaging 9:43 pace. In fact, my pacing was really consistent overall.

My speedwork sessions are also encouraging, averaging under 8 mins per mile for long reps. I am noticing that I am able to maintain a faster mile pace than ever before. Which is encouraging me to think big.

The only thing standing in the way of a sub 4 hour marathon time is me. If I don't train for it, I'll never do it. So I am starting to believe. Maybe I can do this sub 4 hour marathon after all.