So, this probably came up already thousands of times, though here I am. So let me explain...
I have a story I want to tell. I am planning for it to become a novel, possibly multiple novels because of its scale. It's quite a long ride, and I have the characters planned out so far, and I know where everything will go to. But now to my problem:
My story takes place in a "crapsack world", and starts with the MC wandering through it in search of something. So far, so cliché. The thing is, as the reader later on will discover, the world was not always that way, but the MC knows how it was before, and even lived in that world, which looked very much like our own. What's more, he witnessed the event where everything went downhill, where he (and everyone else) was betrayed by a person he had called a friend, and so he swore revenge and to try to revert the results of this event. The whole stuff before this turning point could probably fit into its own book. There is a whole world and society to describe, other characters (of which only two or three will even be alive in the main book I am wanting to write), and much character development for the MC. Especially how the event twists him into the Person he will be for the main part of the Story. But...this is not the story I want to tell.
What I fear is, either I start with this backstory, writing a whole book possibly, for everything to turn bad, and the next book starts with a whole other premise, or I start after this time skip, and introduce the backstory through dialogue, or flashbacks of some kind, which just doesn't fit the scope of this disastrous event.
So my question is, basically: If I as a writer have a backstory that is so long it could fill a book on its own, but is not the story I want to tell but merely results in the main story being there to tell, but still is essential because of this fact, how do I go about it?
Thanks in advance for any suggestions!
Edit: Wow, many great answers and suggestions here, and very hard to pick one. Thank you all for your quick support! As Amadeus' answer is very thorough on the general problem of "how to start a story" that also tackles the question regarding a long backstory, I accepted this answer. The other answers are also great, though, and gave very good suggestions and I wish I could have accepted all of them.