In my own novel I have two settings:
- The (more or less) modern world with some historical backstory but 100% fictional characters. Set in the U.S., the country I live in.
- Ancient Egypt as based on Biblical accounts (so not really history, but still trying to be true to the source material) using a mix of "real" characters and fictional ones.
I can't say my chapters in the first setting were easy to write. But the research and setup for them wasn't very time consuming, and the characters and their actions flowed pretty easily.
Setting number two is another thing altogether. The research is insane (I'm comparing it to doing 20 b'nai mitzvahs, but I think that's an underestimate) and I'm constantly afraid of stepping on historical fact (things true of the society of that time and place) or Biblical canon.
I just started writing again after a few months spent agonizing over character lists (names, ages, relationships, and so on) and am now getting bogged down again in details of everyday life.
At some point, you just have to let go and let your characters write their story. Research and accuracy is important to me, so letting go isn't something I do well. I'd rather get it right now than have to change it all later (sometimes even a small detail completely changes a scene).
But you've done your research. Pull your characters together (the real ones and the fictional ones), outline the basics, and go write.
You won't be able to just parrot the accounts you've read. Nor do you want to make up stuff completely. But maybe you can imagine a character who is there with them, the people who wrote the accounts. Not someone mentioned in the accounts, unless it was brief. What did that person see? Or you can make up a character who is an amalgamation of other people who were really there.
Don't try to write a biography, unless that is your actual plan. Don't speak for someone who has already written her/his story. Speak for the person who didn't write an account. The person not recorded by history. The fictional character who still has a story to tell.