I'll chime in with my first-time perspective, if you can stand more from us! I'd always heard, too, that the marathon was a totally different experience... but for me, it wasn't at all. I trained for it and the effort felt equal to training for and completing my first half-marathon, only, you know, longer. Now, I wasn't racing for time, and "finish" was my only real goal, so I'm sure that contributed. And I went in with the idea that I'd simply go for as long as I felt like going - and if that wound up being 13 miles, or 17 miles, or the whole thing, it would be fine. At mile 25, I was 100% OVER spending so much time with other human beings, but physically felt fine and knew without a doubt I could have done Goofy or Dopey. If someone had been standing at the 26 Miles marker with a registration form, I'd have signed up for marathon #2 right there! Finishing was cool and I was proud of myself, but completing a marathon wasn't life-changing or even particularly emotional for me. It's different for everyone!
I guess my advice is to make it what you want it to be and make goals that make sense for you. It's your race and you get to call the shots!