My first too and I tore up my 3:45 pace wrist band in disgust and did the walk of shame for the last 6 miles head down. I feel that because I didn't run all the way I failed the marathon and there are threads elsewhere here that will agree. Having friends, family and people at work telling me I did well to do it in 4:24 and they could not have done 1 mile and at least I finished does not help. I entered to run. Failed to do enough training outdoors because of the cold weather and long runs on treadmills are not the same thing. My quads just stopped firing. Totally gutted. The only thing is that I can hopefully learn from it. So yeah you have to have been there to understand and there were a fair few of us looking like we had only evolved into bipeds that morning.
Talking to others I think it is very common for the first 3 miles to be a slog then as you say you just hit your stride and your legs go into auto pilot. I've never found anything helps except knowing it will go and just getting through it (character building distance running).