Carl, let's run through the basic outline of that MnM game.
Beginning: narrow.
I start with all seven PCs facing one another in a late night meeting at a restaurant. I don't care about how they got there, because I want to dive immediately into roleplaying. Unless they want to turn around and leave or immediately start a fight, they really don't have much choice but to start roleplaying - and I've written the character sheets so that they'll want to do so.
After 15-20 minutes of talking and roleplaying, just as the talk starts to slow down a tiny bit, I hit them with enemies smashing through the roof to arrest them. This is also narrow; by definition, it occurs either just before the group plans to leave the restaurant, or just before the game bogs down.
The fight is exciting, as there are a lot of hazards (falling off a 120 story building, for one), and NPCs in the fight supply them with crucial information to drive the plot. After the fight is over, I shunt them away by raising the threat of many more enemies arriving in less than a minute - time to escape and make their plans.
Middle - wide.
The PCs are superheroes, so they can go virtually anywhere to make their plans. Also, their plans may be remarkably varied. That's cool - I'll let them retreat to someone's secret lair and figure out tactics, or teleport to a peaceful desert island, or whatever they like. I'll feed them information that affects these tactics, and let them come up with a strategy that negates their opponent's big advantage. (Hopefully, this strategy involves super-science!) This is also a time for role-playing, and more character secrets come to light.
In truth, the only things that are relevant here is exactly how their actions affect the final encounter. They may do things that make this final encounter easier or more difficult (probably involving an orbital space laser), but they know where their foe is. I introduce her secret plan and the serious time pressure that comes with it; during this stage the PCs discover that they only have a few hours to get ready. This stage ends when they attack the villain's complex, in whatever manner they think is most efficient. They can do whatever they want until that time runs out.
End - narrow
My mental image of the villain's complex has only 3 or 4 known areas in it (with a few more that can be added on a whim), and they're all connected serendipitously. In other words, the areas are where they need to be physically at the encounter's start. If I want the PCs to see the biolab first, then they'll blunder into it whether or not they enter from the front entrance, the rear entrance, or through burrowing. I'll provide a flavor combat challenge between the biolab and the Hall of Ego, and the rest of the main fight takes place where I expect it to (unless the PCs do something clever to move it.)
It's important to note that there are definitely penalties for losing or not stopping the villain by the known deadline. The PCs can fail. But I've made it most likely that the final encounter will occur in a known cool location, and the middle part of the adventure just makes that final encounter (and the final outcome) easier or harder.
I'll also point out that "narrow-wide-narrow" is especially well suited to high level adventures, although that last "narrow" might get hijacked! That's because with high lvl D&D, you never know how they're going to get someplace, you only can figure that they will.
