Running

Yeah don't worry about running too fast during training. An occasional parkrun to gauge fitness and mix things up a bit will do. But I would also schedule one or two races as interim goals, maybe go up the distances, e.g. a 10k, then a half marathon something like 4-6 weeks out from the marathon. This'll give you some focus for your training rather than feeling like it's a long slog all the way to the big one. And you might learn things from running with a bib and other people, e.g. keeping a lid on the adrenaline and inevitable over-enthusiastic starting pace.

Besides that, I would pick a decent marathon training plan to follow. It doesn't have to be done to the letter (and the odd skipped schedule due to niggles, feeling too tired or life just getting in the way is OK) but it will give you an idea how to build up the mileage over a fixed period, and reassure you that you're doing it right. Some online plans are overly complicated with hills, interval sessions, etc. but something like Hal Higdon's plans are straightforward enough.

Finally, there's no harm in looking beyond "just get round" and having some sort of time target in mind, but focus on training at the right intensity and you might surprise yourself how much your pace improves just from a few weeks of increased training volume. A half marathon a few weeks out will give you a reasonable idea what you can aim for in the marathon, and more importantly give you an idea on a pace which won't cause you to hit the wall.