You go for it.
Never forget, that if you appear confident, seldom hesitate, and seem like you know what you are doing, the players will believe almost everything you tell them.
You tell them you have a campaign ready and look like you do (you’re carrying a notebook lets say), then they will probably believe you.
Now, the first thing I do in such situations is character generation. Always good for sucking up time and ideas.
While this is in process, Ask every player about what they want in the game. These bits can be cobbled together for future elements.
When each player makes a character, assign them a place in the world. So, the Ranger is just not a Ranger… They are a member of the Order of Oaks. They work for the King, and sideline for the Druids.
The first person done with their character becomes the keystone character. So if they are a Ranger (who is a member of your newly minted Order of the Oaks), connect every new character to the keystone. So the Druid character has been working with The Ranger, and is assigned as a liaison to the Order. The Fighter becomes a squire or knight assigned to the Order or the Ranger’s brother (who obviously gave up his first son status to become a Ranger). The Thief… well nobody knows he is a thief… except maybe the last character rolled up, who is related to him and they are both minor nobles).
This is just fast and dirty character weaving. The player gives you a little bit, then you give them some serious hooks that tie them to each other and the local area. The players can work out the details and try to make sense of it all. (Heck they make sense of randomly rolled stats and abilities, they can come up with a back story).
All the while this is happening, nod and smile like this is “exactly what you want”.
The scenario is as follows… come up with something banal that The Ranger has to do… the group comes along for the fun (make it a local festival…). While the characters are expecting nothing more than to have fun, the players all know that something will happen at the festival. So don’t disappoint them.
Make the mooks interesting or odd. Have them search for something and not quite find it or have them find it, but one of the players kills the mook that has it, and the others try to get it back. Eventually, the mooks all die or run away.
Now you have an action scene, a dark conspiracy, and a mystery, and you have no clue as to what is really going on. Yet if you keep up the brave face, the players will think you do… and begin to speculate… (which you can scoop up and use… making them feel so smart for figuring it all out).
Now, if the session ends, then you have a week or so to actually make it all work.
If not, have the King make an appearance soon after the carnage begins to clear. The King (or suitable noble) commands his Ranger and his vassals to find out who was behind this heinous attack. Now you motivate everyone.
So general advice…. If not, keep being vague and mysterious. Drop something that might be a clue in their way. Make the clue lead them to something they have to travel to. (you have a map, but nothing on it… their traveling will help you fill it).
Some additional advice useful to you in such a crunch
Its like baking a cake (World Building)