Like in any campaign, you should start small and expand from there. You should have some sort of outline where they players should go and how long it should take, but spend your time on what is going to happen today and next week before planning the final encounter with the BBEG. Plan the town and local landmarks around the town, plan the first dungeon, plan a few NPCs around that they will encounter. Don't worry about drawing great maps unless your players are going to see them. I used to do this and would waste my time when a stick diagram would work just as well for showing how the rooms connect. Depending on extra time each week you can expand things like NPC list and a list of names to keep around. Always keep track of where you are heading so plans can be made to keep things on track. Also, you always want a spare fight planned for when things go wrong or become stale.
Another great tool is to plan encounters or details that only become evident later in the story. Maybe there is a symbol or puzzle that they cannot decipher. A few levels later the party comes across a book with the answer in it and now can go back to finish that part of the quest. It is a bit like a video game, but works fine and many players like to "ah-ha" moment.
