Hmmm, we're probably the same time type of writer that thinks of a great ending but has no idea what to write in the middle. What I do there is write down what I need for that ending.
So, example: Kill the demon lord. We'll need: 1) Legendary sword, 2) magic scroll, 3) new spell, etc. To get there: 1) find the legendary sword - for finding the sword, need a few chapters in this or that magical forest. For the magical forest, I first need a chapter or two to find a guide. And for the magic scroll... and so on.
It becomes easy to make a checklist of this one, then work backwards from that. Once you got a long list of things to happen you can start slotting them in chronological order (or if you want flashbacks and stuff). Hope that helps.