Adventure development question

[Cleverdan22]

So I have recently become a DM, and my players are loving the game. I started them out as first levels, and I have been making the adventures. For a while its going to be easy, just writing interesting local stuff for them to get involved in and earn some XP, but down the road I plan to make them larger, getting them involved in a war, making them important. However, I was just curious about how many DMs have done related things and how they have done it. As in, how did you carry your group of 1st level newbies to be heroes of the time?

[AslanCross]

My very first campaign was sort of similar, but I started at 5th level and did intend to take the PCs to 20, but due to time constraints, 10th was the furthest we got.

The setting was Cormyr, in the Forgotten Realms. Relatively stable, respectable kingdom in the middle of some very unsavory neighbors. The primary villainous organization was the Zhentarim: essentially a mercantile consortium of wizards bent on dominating the region through arms and trade that is sponsored by the church of Bane, the God of Tyranny. The de-facto leader of the Zhentarim is Fzoul Chembryl, Bane's high priest.

The villain in the story, however, was not Fzoul, but a half-fiend ogre mage warlock who worked for Fzoul as a high-level enforcer against Zhentarim splinter elements who instead preferred to worship the God of Madness, Cyric. However, Grimgashr believed himself to be destined to not only supplant Fzoul but to become Bane's avatar. Bane's avatars have a bad history of being slaughtered in Faerun, so he wants more security now for his new dark messiah, so to speak.

Let's get into that later. Here's the series of adventures:
1. During Shieldmeet, the primary festival in all of Faerun, the PCs, who are in the capital Suzail, are summoned to the palace. They are hired to free a remote garrison on the border of Cormyr, which seemed to have fallen silent all of a sudden. They discover that it had been taken by hobgoblins, and they seem to be massing in the area. Furthermore, the hobgoblin cleric with them is not one of Maglubiyet, the traditional hobgoblin deity, but Bane.

2. The PCs are awarded the borderlands as a barony, and the garrison they freed as their own base. Before this becomes formal, though, the PCs are sent to purchase some rare reagents for a divination spell that would reveal more about the evidence that the PCs have taken from the garrison. Instead of finding a friendly caravan, they discover a Zhentarim caravan camped out outside the city. The PCs are attacked immediately, but not before they see three suspicious figures: two cloaked humans on horseback, and a third character who dresses up as a slave, but seemed to have been giving orders to the two cloaked men in Infernal. The caravan quickly leaves after the minions attack the PCs, and eventually the PCs catch up with one of the cloaked figures---a woman, a swordswoman. (see image in my sig). She murders one of the PCs (the player quit the game), and she ends up becoming a thorn in the PCs' side for a while.

3. The PCs get back to the city for the formal nobility induction ceremony/ball. Before anything happens, though, they are attacked by masked drow and other assassins. The Lord Mayor of the city and his family are taken hostage, and the adventure ends with a vicious brawl against two assassins: a tiefling martial artist and a black-furred rakshasa. This takes place in the Mayor's mansion's ballroom, with the Mayor and his family hanging helplessly from the ceiling. After the PCs clean up the place, they catch a glimpse of the overtly suspicious, gaudily-dressed nobleman whom they've seen hamming it up with the princess regent: Viram, the Marquis Eldershade. Despite him looking overtly suspicious, he doesn't ping as evil on any detection spells.

4. The PCs are summoned back to the palace for an emergency council with the nobles due to the attack. Things get chaotic in the assembly, especially after the Marquis speaks "convincingly" to the princess regent, suggesting they make trade agreements with their traditional enemy neighbors, Westgate and Sembia, neither of which are well-intentioned against Cormyr. The PCs realize that there is something wrong with the regent--she's been the subject of a suggestion spell. However, the document regarding the agreements has already been sealed, and by law, may not be repealed. (Weak narrative point, I know) A riot then breaks out in the square outside the castle.

5. The sorceress Caladnei, the princess regent's best friend and advisor, knows what's going on and suggests that Eldershade is behind all this. The PCs do some research and discover that Eldershade is likely not who he claims to be---he's actually a member of a disenfranchised noble family whose members were executed for treason. He's back for revenge, but he's not doing this alone.

6. With a small detachment of Cormyrean soldiers, the PCs set out towards Eldershade's castle in the borderlands in the eastern end of Cormyr to root out his massing army and, hopefully, Eldershade himself. They discover that he hasn't been acting alone at all; hobgoblins and draconic creatures are hanging out in the castle as well. The PCs decide to take out the castle, which is a formidable fortress on high ground, with stealth and surprise. Within they discover elements of Bane's clergy, as well as darker powers at work.

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We only got to play until this point. Since I knew we weren't going to get to level 20, this was what I had planned until lv 12:

7. The PCs fight Eldershade (a beguiler) and one of Bane's clerics on the caste roof, along with some dragonspawn. Just as the PCs are about to close in on Eldershade, an airship swoops out of the sky and picks him up---only to turn around and drop him onto the roof in front of the PCs. The old Marquis dies messily. Grimgashr makes his appearance, and challenges the PCs to fight him when they're ready, in the skies above Cormyr's capital city.

8. Within a month's time, Grimgashr's airship appears in the skies over Suzail while hobgoblin armies pound on the capital city's walls. The PCs storm the ship, hoping to take Grimgashr out once and for all. An enormous glowing pentagram forms around the ship---the ritual to summon Bane's avatar has begun. The PCs encounter Grimgashr's fiendish minions and concubine Erinyes, his two lieutenants Caella the swordswoman and Loroshyr, a masked drow wizard, and finally Grimgashr himself. Little do they know that the final component in the ritual is the blood of five righteous souls---the PCs themselves. Huge fight in the air breaks out on the ship's deck.

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Original "to level 20":
-The PCs lead the Cormyrian army on a series of armed interventions against incursions all across the border. The dragonspawn work with the Zhentarim army because they bought out a powerful blue dragon who lives in the Anauroch desert north of Cormyr, and he brings his co-conspirator dragons to battle the PCs as well.
-The PCs discover that an undead contingent is being raised in Eveningstar a city north of Cormyr, near the Haunted Halls, an ancient dungeon. They discover that the contingent is lead by their former party member, the fighter/swashbuckler who was murdered by Caella in part 2, and that she's been raised as a vampire. The adventure ends with an epic clash against the vampire, her lich lieutenant, and two megaraptor skeletons in the heart of the Haunted Halls---an ancient abandoned cathedral of the dead sun god Amaunator.
-The PCs return to Suzail only to see that it has been completely surrounded by hobgoblins, fire giants, and dragons. Begin the siege of Suzail. Part of this involves fighting the hobgoblin king and queen (a high-level warblade and wizard, respectively), as well as a Fire Monolith---a fire elemental of Godzilla-like proportions.
-The PCs and the Cormyrian army fight valiantly, but when the Wyrm Blue Dragon on the Zhentarim's side takes wing, the army practically scatters.
-The drow wizard battles the PCs on top of the city's wall. When the PCs finally defeat him, his contingency triggers: A Reality Maelstrom. It sucks the PCs to another random plane.
-By the time the PCs get back, they realize that Cormyr has fallen, and have to retake it themselves.
-They battle the Wyrm blue on the roof of the castle of Suzail, and then finally Grimgashr as he tries to perform his Avatar ritual in the caverns beneath the castle.