26.2 miles is a long way, and you should be proud.
You went off too fast - and that's a real killer.
You have only been running a year - yes, lots of people do it, but to do yourself justice in a marathon at whatever level, you need to be running longer than that.
Having "only one pace" will change with experience - parkrun is great for that (amongst lots of other things parkrun is great for).
I'd recommend going back to basics. Long term, consistent training, with a gradual build up of distance, will give you a solid base. Give this at least 6 months (as you have already a year under your belt). Keep the longer runs *really* slow - as in "I could almost walk faster than this", and increase 10% a week. Then find a target race (never again for the marathon? Ok, find a half you fancy) and a training plan.
You have the determination - you finished didn't you? Now you just need the training.
ooh loving reading all these, we were watching at just past the half way point and I was trying to see if I saw anyone cheat where we were, I saw a few running run to the loos at the Texaco garage which were on the opposite side of the road, which I'm guessing then if they came out instead of carrying on to the Isle of Dogs they have just stayed on the same side and headed for the finish which would explain why they had missed the mats
Aha! I've always thought if you wanted to cheat the best way would be to pretend you needed a poo. Much better than just ducking straight under the barrier.
And then that Radcliffe woman comes along and just poos at the side of the road, spoiling it for everyone who doesn't want to run the whole way. Spoilsport!