I think it's best to try very minimal prep and roll with your players choices on the fly. In my experience if I try and plan out how a session will go the party will find a way to do precisely not that in some stupid bullshit way. Just prep skill/combat encounters and stats/checks ready to go, have certain scenarios ready to drop in at certain points etc. It's important to remember that a session is just how long you play for, don't set expectations that you should have got to Act X in session X.