The first step will be to beef up the backstories and world creation. For the first draft, I came up with just as much as I needed to write the scene I was in and no more, but now I have to go through and make sure all that stuff is up to scratch. The next step will be to look for thin spots in the first draft and see where I can flesh it out. The first draft came to approximately 69,000 words in 19 chapters, but the fantasy market as it currently stands regards that as being unsellably short (I'll rant about that some other time). If there's another 30,000 words to be found in this story, it's going to be in breathing more life into the supporting cast.
Thing is, when I was outlining this book, I was aiming for "60,000-80,000 words" and I hit that target spot on, not realizing that the fantasy market had moved the goalposts since last time I looked at it. I sorta wonder now if it would be easier to start fresh with a new outline based on a new target of 100,000 words, than to try to graft 30,000 more words into the manuscript I've got. I'll ponder this as I work on the backstory stuff.
Even if I end up going that route, nothing is ever wasted. The first draft of this book is the longest item I've written to date and also some of my best work. So I'm not worried about that! It'll find its useful place.