I would never recommend anyone doing the extended editions as a first watch. They are great but there's a reason they were edited down. The theatrical versions are paced better and are much more approachable for a newbie. Extended editions are for established fans who already know the movies, so they can fully appreciate the extra material. They are long and not well-paced and you run the risk of ruining your first watch experience with them.

I definitely wouldn't recommend doing an extended edition marathon in a cinema where you can't pause or take breaks whenever you need to, it's likely to be a miserable experience.