My dad ran one full marathon and my friend is about to.
My dad did a proper training, my friend is kinda just winging it, but it's basically always the same ingredients:
- enough sleep and health, maybe a checkup beforehand
- training up to like 20km weekly (can't remember the exact figures)
- eat lots of complex calories; pasta, potatoes etc are great
- do one practice run for marathon distance to get a feel for it
- train right up until marathon, the exhaustion from previous training will physically prevent you from starting off to fast
- be consistent. The tempo doesn't really matter, but no walking, only running. Taking breaks by walking will throw you off more than you expect, trust me.
- maybe check your shoes and your running style with an expert (maybe at a shop for sports gear), it can help a lot to wear the right shoes and reduce additional issues caused by a bad running style or subpar shoes.
if you do even halt of the things and you're fit to begin with you'll probably be fine.