Even when my group plays every week or every other week, I try to have a solid beginning, middle, and end to every session's action. Every group is different, of course, so YMMV as they say. I'm not sure how similar my approach is to your "one-shot" approach. Here are some ideas.
1) Watch the clock. Not obviously, of course. But be aware of how quickly time is passing and start pushing the group when necessary. For a four-hour session, the first hour is the beginning, the second and third hours are the middle (with a surprise at the end of the second hour), and your climactic ending is the last half-hour. That only adds up to 3.5 hours because inevitably you'll run over, add stuff in, miss a player who's putting the baby to sleep, etc.
2) Trash the map. By all means, use a published adventure. But even in the 32-page adventure styles you mentioned, you should delete (many) rooms and scenes as necessary to crunch it in to a single session. Don't worry about "missing" a sense of verisimilitude. Just put in the most interesting rooms (interesting to your players, not to you) and the critical plot points. By far the most likely result is that your players will love the increased pace.
3) Think of pace, scene types, and surprises ahead of time. For example, try to start with a combat (or chase or something action-oriented), then a slower dialogue scene, then a combat scene, then a surprise plot twist that involves one of the characters' backstories, then a puzzle, then a future plot hook that involves one of the characters (maybe a portrait on the wall showing the BBG at the end of this adventure wedding a PC's mom), then a combat, then the climax. But...
4) Don't railroad the PCs. Let them go where they like and do what they want. But what if their actions lead to three of the same scene type in a row? Well, if your PCs just fought a manticore pack, then chose to fight through the black pudding and myconid hatchery, and are now walking down the hall to the door where the adventure says an insane burn-the-world efreeti noble waits with a chainmail bikini blonde in one hand and a monkey-gripped falchion in the other, lusting for combat, you know what? That insane burn-the-world efreeti noble becomes a riddling, puzzle-loving, brass-pince-nez wearing, chess-for-hostages-playing efreeti noble just before the door opens.
5) Run the climax. As you approach your final half hour, run the climax. Bring it to the PCs if necessary: the BBG heard about his dungeon being torn up and he comes to face the PCs. Or, if one of the PCs rolls a natural 20 on any skill roll at the right time, make a huge ZOMG!!!1!! face and reveal something as a result of the roll that leads the party straight to the climax. In other words, seize on something to make the deus ex machina seem more reasonable. A nat 20 on a heal check? The cleric notices that the basilisk's claws have traces of desert sand found only in one particular oasis; that must be where the BBG is! Don't worry about contrivance much; your group has already agreed to play full sessions in 4 or 5 hours so they are going to be willing to overlook a bit of contrivance in the name of that goal.
And like Robilar said, above all, have fun!