Let's start with a few assumptions. I'm assuming you're not playing a sandbox campaign. And I'm assuming your campaign takes place in a relatively traditional RPG where you have a DM who presents a scenario to the players, who respond to that scenario.
From your description, it sounds like you have an adventure that looks something like this:
- (The orcs kidnap a bunch of people.)
- The party arrives on the scene (the game starts here).
- The party fights a bunch of orcs to free the townsfolk.
We'll add on to this as we go along.
Step 1: Standard Padding
If you need to pad out an adventure, there are two very easy wells to draw from to add an extra encounter or two:
- How do the players find out what they need to know? (investigation)
- How do the players get where they need to go? (travel)
These are useful to call out on their own, because they can be added to pretty much any adventure. It is very rare in role playing games for the party to have all of the information they need. It is also very rare for the party to spend an entire adventure sitting in one place.
Investigation
Investigation is really your chance to provide exposition. Think of the things the party needs to know, and then figure out how that can be conveyed through things they find or interact with. Some information you give out for free, and some may require skill checks or role playing to get at.
In your case, here are some ideas:
How does the party know people are missing? (there aren't enough people here; buildings are damaged, and there's even the occasional bit of blood. Talking to the townsfolk makes it clear that they were attacked)
How does the party know it was orcs? (investigation checks reveal broken and discarded weapons of orc manufacture)
How does the party know where the orcs went? (the villagers can give a vague sense of direction; tracking can reveal their trail; investigation or knowledge shows signs that these are the Orcs of the Red Right Hand, known to live some distance to the west)
A word of warning: It is very easy to write a challenge that looks like "the players must make a DC12 tracking check to pick up the orcs' trail." This looks fine on paper... Right up until the players botch the check, the orcs get away, and the adventure ends to the sound of sad trombones.
When dealing with investigations, make sure that the players have enough information to proceed for free. What good are the checks then? Give them other benefits... For example, failing the check could mean arriving after a third of the hostages have been executed. Or after the orcs have dug in.
Or fail forward -- Use the failure to move the plot along. The players fail the tracking check, and end up on the trail of a rival party of orcs. There aren't any hostages here, but there is loot and information (or an opportunity for clever roleplaying).
Travel
No matter what else the players do, they need to get to the source of conflict. This gives the DM plenty of opportunity to interact with the party... Whether it be by showing glimpses of their antagonists (orc scouting parties!), giving them an encounter with the local fauna (territorial dire wolves!), hazardous terrain (brave the treacherous mountain pass!), or even more routine rolls for tracking, survival, or maintaining a reasonable pace.
This is an opportunity to add foreshadowing, or just kill some time.
Let's add a bit of padding to our adventure:
- (The orcs kidnap a bunch of people.)
- The party arrives on the scene (the game starts here).
- The learns of the attack and the hostages. If they talk to the mayor, he'll offer a reward.
- A bit of investigation can turn up the fact that these are the Orcs of the Red Right Hand, whose stronghold is east of here.
- While en route, the party discovers that a ravine they need to go through has been blocked by an avalanche. They need to get around it to proceed.
- The party encounters an orc scouting party. If they did well on their tracking and other earlier rolls, they spot the orcs first... Allowing them to ambush their foes, tail them, or do other clever things.
- The party fights a bunch of orcs to free the townsfolk.
Step 2: Ask Questions with Purpose
Asking about motivations is well-known advice. But it's important to remember why you're asking the question. You wouldn't answer the question "how was your day?" with "72 degrees and sunny," because that doesn't address the purpose of the question.
The same is true here: your purpose is to find interesting twists, complications, and information for the players. You aren't the orcs' therapist.
"The orcs don't want them as slaves" is an answer to "what are the orcs' goals?" (well, part of one perhaps...) but it doesn't move your design session forward. Give it some more thought, until it does. Maybe the orcs are looking for ransom (in which case they'll need to communicate with whoever has money and keep the hostages safe, but will probably be well prepared for pursuit). Maybe the orcs are looking for sacrifices to their dark god (in which case the party is on the clock!).
What are some questions that might make this interesting? Well, I rather like that "sacrifice to their dark gods" bit. What about "why is the mayor offering a reward?" Hm... Perhaps his children have been captured by the orcs?
- (The orcs kidnap a bunch of people.)
- The party arrives on the scene (the game starts here).
- The learns of the attack and the hostages. If they talk to the mayor, he'll offer a reward for rescuing his children.
- A bit of investigation can turn up the fact that these are the Orcs of the Red Right Hand, whose stronghold is east of here.
- While en route, the party discovers that a ravine they need to go through has been blocked by an avalanche. They need to get around it to proceed.
- The party encounters an orc scouting party. If they do well on their tracking and other earlier rolls, they spot the orcs first... Allowing them to ambush their foes, tail them, or do other clever things.
- The party fights a bunch of orcs to free the townsfolk. Unless they've done particularly well, this will be at the fortified stronghold at the orcs' sacred place.
Step 3: Split Things Up
You don't need to fight a whole horde of orcs all at once. We've already added a few orc scouting parties, but how can we get a bit more mileage out of these orcs?
We'd also like to give the players some choices, and allow them to have a partial success even if they aren't completely successful.
So let's say that when the players catch up with the bulk of the orc raiding party, they discover that the orcs' leader has gone on ahead with the choicest sacrifices (naturally including the mayor's children).
What was one encounter (fight a bunch of orcs) is now two: Fight a bunch of orcs, then chase and fight their leader. The players now have the opportunity to decide whether they should press on after the main horde, or rest and heal their wounds. And if they fail to stop the chieftain in time, they'll still have at least saved some of the hostages.
- (The orcs kidnap a bunch of people.)
- The party arrives on the scene (the game starts here).
- The learns of the attack and the hostages. If they talk to the mayor, he'll offer a reward for rescuing his children.
- A bit of investigation can turn up the fact that these are the Orcs of the Red Right Hand, whose stronghold is east of here.
- While en route, the party discovers that a ravine they need to go through has been blocked by an avalanche. They need to get around it to proceed.
- The party encounters an orc scouting party. If they do well on their tracking and other earlier rolls, they spot the orcs first... Allowing them to ambush their foes, tail them, or do other clever things.
- The party catches up to the main horde. If they've done well, they catch them before the orcs reach the stronghold. If they do poorly, after (more orcs! defensive structures!).
- The party chases down the chieftan, and rescues the remaining hostages.
That should be serviceable for a simple adventure. The players will need to stay fairly close to the rails, but it should keep them occupied for a few hours.
Step 4: Branching Out
Once you have a framework for your adventure, it's time to start branching out. What happens when the players react differently to a problem? What happens when they fail at a step? How can you flesh out the background information?
For this I tend to use mind maps. They're quick, and you can quickly see which areas you've put a lot of work into, and which ones may be lacking.
In this adventure, I might make a node for town. Connect it to a node for "the mayor is offering a reward," connect that to a node labeled "why," which ultimately connects to a node labeled "his children were kidnapped."
For any given encounter, I ask myself questions... Why are things the way they are? How do the players know? What is another way the players could come at this? What happens if the players fail at this point?
If a question comes up that I don't have an answer to, I can just leave that node hanging and move on with other parts of the map.
My goal with these isn't to write down a complete adventure, just the bullet points. I have a good memory and run my own adventures, so extensive notes generally aren't necessary. If you intend to publish your adventure, you can transcribe it later.
Here are a couple of examples from my notes. As you can see, they don't have to be pretty :)
Appendix A: Reading
A big source of ideas for adventures comes from stealing those ideas from other works. Do enough reading, and you'll find yourself thinking "this reminds me of that time when..." Books don't map directly to RPGs, but a good idea is still a good idea.
Your dungeon master's guide (or similar work) will likely have a list of recommended reading. Go out and read them.
I also happen to have just watched a YouTube video on the topic. It has some good suggestions, if you're playing in the realm of heroic fantasy.
Appendix B: Adventure Fronts
Since you mentioned them in your question, a word on adventure fronts.
Adventure fronts are awesome. But they aren't adventures... They are an idea for an adventure. An adventure front for everything above would basically say "There are some orcs kidnapping people over there. If you don't stop them, they'll summon a demon."
In Dungeon World, the idea is that you do a bit of prep, and then build the adventure on the fly with the input of the other players.
They are tools for simulating a living world, and less so for writing a pre-planned adventure.