Don't read if you've not yet run! It might not be what you want to see. 
I finished my first marathon on Sunday (Brighton) and I'm finding myslef really disappointed. All the races in the build up, the long training runs suggested that I should have been on for nipping under 5 hours. And I didn't.
Set out fairly easily, didn't get carried away, but settled down at low 11:xx min miles. Didn't have a pace band or anything, just concentrated on the mile I was running. It seemed to be going OK, some slower, some faster, but nothing dreaful. The undulations are in the first half, so I was feeling pretty pleased with myself. Half way in 2:26.
I always race on manual lap, so there is always the possibility that it will go wrong - as this set of splits will prove. * show where it's 2 miles & I forgot to hit the button.
11:51, 10:54, 22:18*, 11:12, 10:54, 11:24, 10:45, 11:21, 11:08, 22:09*, 11:10 (to 13 miles - half 2:26)
11:09, 11:36, 11:45, 12:38, 11:06, 12:16, 25:44*, 24:34*, 11:58, 12:54, 13:26, 2:12
But, and this is the bit I think I'm most upset about - I just couldn't keep it going. Had a slow mile 15, when we turned back into the wind, then stopped and walked in mile 16. Nothing in particular hurt, it all just ached. Did some calculations and worked out that 5 was still on, if I could maintain 12:00 to the end. Well I consider 12:00 my easy "I could run all day" pace - but I couldn't even manage that.
I found the stretch from 20 to 23 just awful - and not helped by being knocked almost to the floor by a runner comming the otherway on the wrong side of the cones. Maybe I should have been looking where I was going & got out the way, but I was in my own private patch of hell and not looking too far ahead so I didn't have to see how far I had to go. I did salvage some pride and "run" (I use the term loosely) the last mile and found some pace for the last 0.2. But it's small comfort.
Crossed the line and all I could feel was relief it was over. No runner's high for me. I can't imagine I'm the only one who's run a marathon and it not go to plan, nor to be disappointed in a run. I don't even feel I can say I ran it, I'm sayiong I finished it. I didn't run it, I walked some of it - certainly more than I'd ever planned to.
I can see the only thing to do might be to try it again - but I really don't want to have to do that much training again in a hurry. I found it became something really intrusive - it became something that life fitted round, rather than fitting into life.
My plan was to use the fitness and knock of some 5&10 k PBs, which I know I'm capable of; I ran a 5k for the middle section of a tempo run faster than my 5k PB! But at the moment I don't even want to put my shoes on & get out the door. I probably just need a slap, but any words from someone who's been in the same boat would be nice.