porgesomer: What do you mean by optional?
Well, in MM1 there were several fights that I found that I couldn't win, and I never bothered to actually try to win them before finishing the game. There were also a few tougher dungeons that I didn't need to explore in order to win. In MM2, there are similar encounters, but that time I did finish them all, even though I didn't have to... which made finishing the game proper quite easy afterwards. In World of Xeen, there are several whole dungeons that are meant for a challenge only and are not needed to complete the game, nor do they have any quests associated with them. The Xeen games also have many, many optional sidequests.
Depending on how many of these things you try to complete, you can easily add a lot of hours of gameplay.
Oh and about transferring from MM1 to MM2... yes, you can transfer your characters, but you will NOT get their equipment (probably a good thing, because then there'd be no good loot to find in MM2) and your characters' levels and stats will be scaled back. If you officially finish MM1 before importing, you will have your levels and stats scaled back less than if you transfer characters who have not officially finished MM1. Don't worry though, your characters in MM2 will quickly become far more powerful than they ever were in MM1!
A full explanation of how importing characters works is given in the manual to MM2, which you should have as part of your MM Pack.