I don't think you're going too big too soon. Having the mindset to complete long distance running / cycling is a very valuable component. Your mind is too often your worst enemy when it gets rough, so keeping that under control could be the difference... but you already know this.
Just finishing is a good idea, as you'll then feel confident enough to do it again, rather than feeling like it was torture.
As for hydration, I've never done it without drinking, but I don't drink for anything under 32k, so it might be fine but I'm not willing to risk it. I always have 50/50 diluted fruit juice, with a big pinch of salt and some sugar (glucose is absorbed quicker than fructose), as well as plain water and an energy gel just before starting. I probably drink about a litre, but I do drink a lot regularly every day, so hydration isn't an issue for me.