In the Middle of Things

The difference between fiction and reality? Fiction has to make sense.
--Tom Clancy

I have three stories going right now that are only missing one thing each---a middle. I'm happy with my beginnings and I've already written the endings. Now I just have to fill in and get the characters from point A to point Z. It's points B through Y where I run into problems.
Luckily, when these stories formed in my brain, I knew what they were about and how I wanted things to turn out. The problem is getting my story to flow logically and believably from one point to the next.
Many writing-advice gurus would suggest that I not worry about the ending and just let the stories flow organically, but that won't work for these stories. This is mainly because for one the ending felt so inevitable that I had to write it.
For now, I'll just keep muddling through the middle, because the only answer for getting over a rough patch is to just keep writing and rewriting until it all comes out right--or close enough.