The marathon long run

Yes, and you do not need much beyond it. Part of the reason is physical, but just as much is in your head: you want to reach the start line knowing you can run a long way and handle it.

For someone starting out, much past 30 km stops adding a lot and starts to cost you. The fitter and more experienced you are, the further it can stretch, but even most elites rarely run past about 40 km in training, and they do not need to. For most people 35 to 36 km is plenty, maybe 38 when you are deep in it, and there is still a lot of value in a 30 to 35 km run, or even 32. What you are really after is around 2 hours of good running, a bit more for some, up to 2h15 or 2h30. Past about two and a half hours it is mostly diminishing returns and can tip into doing harm. Two hours is the beautiful mark for a lot of runners.

Get them close to marathon pace when you can. Around 20 to 35 seconds per km slower than goal pace is the sweet spot. The fitter you are and the nearer race day, the closer you creep. In a really good block your long runs might sit only about 20 seconds off marathon pace, simply because you can handle it by then.

To get as close to the feeling of marathon running as you can without paying for it. A 2 to 2.5 hour long run with up to an hour of marathon pace in the back half is about the closest stimulus to the race there is, and it keeps the recovery cost low while it does it.

No. The last 10 km on race day comes from your accumulated volume, then running tired, freshness, fuelling and confidence on the day. Rehearsing the full distance just digs a hole you then have to climb out of.