Seeking Advice from the wise- Prequels: How the fudge do I make this fun for me?

I'm a funny kind of writer. I like to keep at least to completed first drafts ahead of myself. So, as a dear friend of mine is checking over one manuscript for errors before I send it off, I have another one ready for editing, and another one to write.

This brings me to my latest WIP, one that I've been kind of putting off starting with the aid of multiple excuses, and really need to start in order to get it off my mind.

 I'm starting a prequel.

Terrifying for me, which is a little baffling. A prequel means that all the characters are there, all developed, the ending I already decided, I even know (more than I do for most of my stories) all the little events that get them to where they are.

The problem? Where's the pay off? I like to discover as I write, living it out right alongside of the character as she discovers little things about herself she didn't know or denied, fall in love, and, in the end, come off a winner in some way. I know how this is going to end, and it's not going to be pretty. And, I think the worst part, I know what's NOT going to happen.

So I seek advice from those wiser than me as to how I can approach this so it's more fun for me (yes, for me, not the audience. I'm selfish this way). Have you written a prequel, either to your own work or someone else's, and how did you manage to get through the journey when you've already seen all the landmarks?