How Long is Forever?

I was very excited to finish this. 
So a week or two ago I randomly started daydreaming about a single scene;
The setting, a keep near a small town where young Earth pony knights are trained. A platoon of Pegasi is stationed there temporarily, but they are usually seen as "chasers of fantasy" and believed to change sides as their whim decides, and are being treated very dismissively, almost to the point were glances at their wings almost treat them as symbols of someone who is ready to abandon the fight at any time.
The scene, as an earth pony, walking down a corridor in the keep and seeing a very cute young Pegasi and deciding to say hello, being nice to her and admitting to actually liking Pegasi.
And the scene just build on more and more until eventually I had an ENTIRE frikkin story in my head!

The Pony Guard, a set of knights that guard this keep, and the Feather Corp, a team of Pegasi stationed there for training. The Pegasi hate staying in one place for too long or settling down, so they are constantly stationed in different locations.
A story involving forbidden(ish) love as an Earth pony and a Pegasus fall deeply in love! but know that one day soon, they may have to end up saying goodbye.
A story of drama and conflict and romance, with an underlying plot creeping in of a mystery that must be solved, a plot festering in the shadows that may put the town, and maybe the whole world in danger.

Honestly it's such an exciting story to me and I really can't wait to write it and I just so so hope I actually can before I lose steam.
This scene takes place somewhere towards the end of the story, and that's all I'll say for now.