The game was designed with an emphasis on 4 players and the levels that are actualy played. This means 1 through 15. The closer you get to the top limit, the more the DM has to do some preperation time. It was true in 1st, 2nd, 3rd, 3.x, 4th and it is still true right now. Some Epic campaigns I did in 3.5 and in 4th were quite "swingy" that I had to add to the stat blocs since all my groups are usualy at 6 and not 4 players.
The way I design campaings is through a reversed synopsis. I know how it will end. But I don't know how it will start nor do I know how it will go there. I write the main story line, the kind of magic that will/should help and fill in the details as the campaign grows.
It can go a bit like this.
Level 18-20
Now the players have to attack the ancient arch devil's sanctum. A direct assault is out of the question and they will require an angelic army to help create a diversion. If the archdevil's plans goes into fruition, he could even become a god in his own right. That diversion, will buy the players about 1 hour to 1 and a half hour of time. Enough to disrupt the archdevil's plan but it will be close as the devil's sanctum is deep within the forteress. Fortunately, the imp's informations and all the scrying made will help the players go through the escape route the old arch devil had designed so long ago. Traps and hazard will be present along the way as well as some guards. All other routes are too dangerous to take.
With that end adventure in mind, I need a hook.
Level 10 to 12
The Scourge lord undead army has grown quite large by now. All these undead that have flocked to his old fortress in the mountains are a formidable force to reckon with. The Scourge Lord is currently tapping some of Myrkul's power to fuel is undead creating rituals. The Dracolich and Death knight servant will make it hard to get to whatever is the Scourge Lord. Lich? Quasi deity? or is he entirely something new?
Level 1 to 3
A tribe of kobold are threatening a small village. Players are asked to find what happened to a few caravans. They will stumble upon some advanced scout from the main tribe. The kobolds are afraid and want to move south. They were chased off from their caverns by "big tusky uglies" (orcs) that have normaly no interest in kobold caves. The players will encounter a few undead moving northward.
At this stage I don't know what kind of characters the players will make. I will only write up the next adventures as I learned the PC's goals, hopes and tastes. I would probably put that adventure in the Vilhon Reach, but the Silver Marches or the Galena mountains north of Damara could work out fine too. It all depends on what kind of characters the players will be doing.
With only these guidelines I get sure that I won't railroad the PC (at least it will be subtle). And with a foreknowledge of what is to come I will be able to prepare and to improvise on what is to come if needed. Obviously I will try to stay ahead of the players with at least 2 to 3 sessions that will have a semi-write up. Usualy some plans, a few planned encounters with new and old NPCs in mind depending on which one the players liked the most. The rest can be improvised on the spot as needed.
All that to say: Over preparation is not good. Underpreparation is worst. You need to find a middle ground that will respect players, DMs and the game itself alike.